LECO: Learnable Episodic Count for Task-Specific Intrinsic Reward
Anonymous Author(s)
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Abstract
Episodic count has been widely used to design a simple yet effective intrinsicmotivation for reinforcement learning with a sparse reward.
However, the use ofepisodic count in a high-dimensional state space as well as over a long episode timerequires a thorough state compression and fast hashing, which hinders rigorousexploitation of it in such hard and complex exploration environments.
Moreover,the interference from task-irrelevant observations in the episodic count may causeits intrinsic motivation to overlook task-related important changes of states, and thenovelty in an episodic manner can lead to repeatedly revisit the familiar states acrossepisodes.
In order to resolve these issues, in this paper, we propose a learnablehash-based episodic count, which we name LECO, that efficiently performs asa task-specific intrinsic reward in hard exploration problems.
In particular, theproposed intrinsic reward consists of the episodic novelty and the task-specificmodulation where the former employs a vector quantized variational autoencoderto automatically obtain the discrete state codes for fast counting while the latterregulates the episodic novelty by learning a modulator to optimize the task-specificextrinsic reward.
The proposed LECO specifically enables the automatic transitionfrom exploration to exploitation during reinforcement learning.
We experimentallyshow that in contrast to the previous exploration methods LECO successfully solveshard exploration problems and also scales to large state spaces through the mostdifficult tasks in MiniGrid and DMLab environments.
Introduction
In reinforcement learning (RL), tackling the problem of sparse rewards still remains a great challenge.
Many works have been proposed to develop dense synthetic reward, which is generally calledintrinsic reward, to provide the motivation for exploration in the absence of extrinsic rewards fromthe environment [5, 24, 1, 26, 13, 36, 31, 10, 38, 27, 35].
Among them, count-based exploration[29, 4, 23, 22, 31, 21, 5] that encourages the RL agent to visit novel states, is one of the most popularclasses for intrinsic motivation since it is simple and effective in many tasks, and more importantly itis theoretically justified [29, 19].
In general, the state novelty can be measured within a single episodeor through an entire task across episodes.
In recent RL problems, environments are often changedover episodes or even procedurally generated within the same task [7, 3, 20, 8, 16].
Therefore,per-episode novelty by episodic count has been widely adapted in modern hard exploration problems[1, 26, 36].
However, in high-dimensional and/or continuous state spaces, state counts need to be approximatedin an abstract space since most states are likely to be unique and novel in the original space, leadingto insufficient exploration [4, 23, 26, 1].
In addition, computation of episodic count at every timestep based on the episodic memory could be burdensome if an episode lasts for a long time and therefore requires fast hashing [31].
Previous works have discretized the state space to obtain the hashcode [31, 14, 9] for a fast pseudo-count, but they have not extensively applied it to recent difficultexploration problems such as procedurally generated and partially observable environments.
Moreover, existing count-based exploration methods have mostly employed task-agnostic statenovelty, and thereby task-irrelevant count may disturb its intrinsic reward bonus to faithfully reflecttask-related information [18, 2, 17, 38].
This problem could be severe in exploration by per-episodecount.
For example, task-wise familiar states across episodes can have high novelty in each episode,which confines overall exploration into smaller areas, while some states that are highly related toextrinsic rewards need to be repeatedly revisited even in a single episode for obtaining extrinsicrewards more densely.
In this work, we propose a hash-based episodic count that is learned to provide task-specific intrinsic motivation in solving hard exploration problems.
The proposed L earnable E pisodic CO unt(LECO) produces task-specific intrinsic rewards that are efficiently and practically applicable tohigh-dimensional states, long-horizon episodes, procedurally generated environments, and highlysparse extrinsic rewards.
In specific, the proposed intrinsic reward is composed of the episodiccount-based novelty and the task-specific modulation.
Our episodic count makes use of a vectorquantized variational autoencoder (VQ-VAE) [32] to automatically obtain the discrete state codesfor a fast counting.
In addition, we exploit a learnable modulator to control the episodic novelty inrelation to task-specific extrinsic rewards.
The proposed LECO also adaptively shifts the RL phasefrom exploration to exploitation.
Experimental results on the most difficult exploration tasks inprocedurally generated environments of MiniGrid [7] and DMLab [3] show that LECO efficientlysolves such tasks and outperforms previous state-of-the-art exploration methods.
Our main contributions can be summarized as follows:
• A novel learnable episodic count is proposed to efficiently provide task-specific intrinsicmotivation to the RL agent under complex environments with sparse rewards.•
A complementary synthesis of learnable hash code based on VQ-VAE and learnable intrinsicreward modulator enables task-specific transition from exploration to exploitation.• The proposed LECO demonstrates consistent performance improvements over previousexploration methods in solving various hard exploration benchmark tasks.
Related Work
Count-based exploration.
Count-based exploration was first proposed in tabular tasks [29], whichpromotes the agent to reduce its uncertainty by visiting unfamiliar states corresponding to low counts.
However, it is basically ineffective in high-dimensional state spaces since every state would bedifferent.
Therefore, several recent works have utilized count-based exploration for high-dimensionalenvironments.
For instance, pseudo counts are derived from density models using context treeswitching [4] or PixelCNN [23].
More recently, RND [5] has approximated the state visitation countby the difference between a random fixed target network and a trained predictor network.
RNDhas also been used in other methods for measuring the state novelty [1, 36, 35].
On the other hand,a number of algorithms have transformed the high-dimensional state to the discretized hash codefor fast visitation counting by a hash table [31, 10].
Similarly, we obtain the episodic count by thediscretized hash code.
However, we exploit learnable hash codes by VQ-VAE that is trained toapproximate the state density.
In addition, different from [14] that directly produces a global code by
VQ-VAE, we concatenate local codes as in the original VQ-VAE for generating a single state code.
Task-specific intrinsic motivation.
While count-based exploration methods have typically utilizedthe state novelty in a task-agnostic manner, curiosity-based methods have tried to consider task-relatedinformation for intrinsic motivation by learning the environment dynamics [24, 27, 18, 26].
Theyhave developed intrinsic rewards based on the prediction error upon the agent’s actions.
Amongthem, recently, a number of approaches have exploited the task-dependent state novelty using theinformation bottleneck in order to ignore task-irrelevant distractions [18, 2, 17].
Meanwhile, therehave been a number of attempts to directly learn the intrinsic reward function to ultimately optimizethe task-specific extrinsic rewards [38, 37, 34].
Their algorithms are generally performed under themeta-gradient framework.
LECO also adopts the idea of learnable rewards and the gradient-based bi-level optimization [38] to construct a task-specific modulator for intrinsic rewards.
However,unlike those works that do not work well with sparse extrinsic rewards, LECO takes a full advantageof intrinsic motivation by the task-agnostic state novelty based on the episodic count before obtainingthe extrinsic rewards.
Exploration for procedurally generated environments.
One of the important challenging RLproblems is robust exploration in a procedurally generated environment where the environment(partially) changes between episodes, and hence the agent needs to explore a very large state spaceand have strong generalization ability.
Many recent RL benchmarks deal with procedurally generatedenvironments [7, 3, 20, 8, 16], and accordingly state-of-the-art exploration algorithms have tried toimprove sample efficiency in such environments.
RIDE [26] has used a change between successivestates on a latent space as an intrinsic reward while AGAC [13] has encouraged action diversityby an adversarial policy.
NovelD [36] has exploited regulated difference of the novelty betweenconsecutive states, and MADE [35] has devised exploration by maximizing the deviation frompreviously explored regions.
AMIGo [6] has proposed a teacher that generates count-based intrinsicgoals.
In contrast to these handcrafted or task-agnostic intrinsic rewards, LECO learns to leveragetask-level information to guide the count-based exploration especially at the episode level due tothe procedurally generated environment.
Moreover, LECO also shows improved performances on
DMLab tasks with higher-dimensional partial observations.
Learnable Episodic Count via Task-Specific Modulation
[Figure] 3.1 Notation
[Figure] Our problem setting follows a typical RL problem described using Markov Decision Process definedby a set of states S , a set of actions A , and a transition function T : S ×
A → S .
An RL agentinteracting with the environment samples its actions from a policy, which is an action distributionfunction π → P ( A ) , and receives a reward, r : S×A → R , upon state transition.
The objective of theagent is to learn a policy that maximizes the expected sum of rewards G t =
E π (cid:2)(cid:80) ∞
k =0 γ k r t + k +1 (cid:3) ,where γ ∈ [0 , 1] is a discount factor.
Among various sub-problems that RL faces, balancing the trade-off between exploration and exploitation still remains a fundamental problem [30].
Especially, efficient exploration is important whendealing with sparse rewards.
One approach to solving such problem is to design an algorithm thatdirectly generates exploring behavior.
More specifically, a widely adopted method is to design anintrinsic motivation through a form of a reward such that the reward which the agent receives at timet is given by r t
= r et + αr it , where r e is the extrinsic reward from the environment, r i is the intrinsicreward designed for exploration, and α is a hyperparameter for scaling [26].
3.2 LECO Intrinsic Reward
The use of state visitation count for intrinsic rewards based on the state novelty has been a coreconcept in several previous studies [26, 1, 36, 13, 28, 15] due to its simplicity, task-agnostic property,and theoretical basis.
However, such state-novelty measured from visitation counts monotonicallydecreases as more experiences accumulate, equally for both important and unimportant states.
Thus,an agent would be discouraged to visit even the task-important states after a certain period of time, andthe exploration towards those states vanishes [1].
Furthermore, an intrinsic bonus based on the statevisitation could lead the agent to only pursue any state novelty, especially in the partially-observableand sparse reward environments.
For example, imagine a task in which the goal is to find a coloredkey hidden in a box and unlock a door that has the same color as the key.
If the box changes its colorto a random color every time when it is opened, the agent would repeatedly open and close the boxjust to obtain the intrinsic bonus.
In this task, the agent should be encouraged to find the door afterobtaining the key and discouraged to repeat task-irrelevant actions.
We directly build upon this ideaby decomposing the intrinsic reward into the state-novelty and task-specific modulation, to balanceexploration and exploitation, respectively.
We design our intrinsic motivation to encourage the discovery of state novelty that is adaptivelycontrolled to maximize the extrinsic return, hence the name L earnable E pisodic CO unt for taskspecific motivation.
Specifically, we define the general form of intrinsic reward as a sum of twoparts,
[Equation]
(1) where r ep t ( s t +1 )
= 1 / (cid:112) N ep ( s t +1 ) is the intrinsic reward from the episodic state-novelty, N ep ( s ) isthe episodic visitation count of state s , and r ta is the task-specific modulation with λ ∈ [0 , 1] as itsweighting factor.
The first term measures the episodic novelty by state visitation count within thecurrent episode that is initialized to zero at the beginning of each episode.
The second term learns tomodulate the episodic novelty in the first term according to whether the state is beneficial or harmfulin achieving the task-specific objective.
Note that the previous action a t − 1 is included as the inputto reward calculation to provide extra transition information.
A schematic diagram of the proposedintrinsic reward can be found in Figure 1.
LECO comprises of two submodules: 1) a learnable hash that encodes input observations to discretehash codes for episodic counts and 2) a modulator that maximizes the task-specific extrinsic return byadaptively scaling the episodic novelty.
Here, the modulator additively scales the episodic novelty.
Compared to multiplicative scaling, the additive modulation allows the episodic novelty to be retainedsolely when the modulator does not generate enough signals due to the scarcity of extrinsic rewards.
3.3 Episodic Count via Vector Quantized Hashing
Previous researches have extended classic state visitation counts [29] to high-dimensional state spaces[4, 31].
A generalization of the classic count-based approach by locality-sensitive hashing (LSH)on the continuous embedding space was proposed [31] where the latent space is obtained by anautoencoder (AE).
The use of a hash table in their approach enables counting the occurrences ina constant time, taking the advantage of scalability, especially in modern distributed RL systems[12, 11, 25].
However, such LSH does not take into account the spatial information due to its dataindependency [33].
As an alternative to AE, we propose to utilize vector quantized variationalautoencoder (VQ-VAE) [32] that directly optimizes the discrete compressed representation of a state,producing a hash code without LSH.
In LECO, a state s t is mapped to a hash code c t by vector quantized hashing based on VQ-VAEcomprised of an encoder, a decoder, and a codebook with θ enc , θ dec , and θ e = { e k ∈ R D } Kk =1 as theirrespective parameters. Here, K is the number of codes, and e k is the D -dimensional embedding ofthe k th code.
In detail, first, the input state s t is encoded to a spatial representation z et =
f enc ( s t ; θ enc ) ,where z et ∈ R w × h × D , and f enc ( · ; θ enc ) is an encoder of VQ-VAE. Then, the representation z et ismapped to z q t by finding the nearest vector in the codebook θ e for each vector in the spatial positioni ∈ { 1 , 2 , ..., w × h } . To be more specific, let us denote the vector in position i of z
et and z qt as z et ( i )and z qt ( i ) , respectively. Then, z qt ( i ) =
arg min e k ∈ θ e ∥
z et ( i )
− e k ∥ 2 , and the hash code is defined asthe sequence of indices such that c t = [ k i ] w × h
{+i =
+}
where k i = arg min k ∥ z et ( i ) − e k ∥ 2 . Finally, thedecoder then predicts the mean µ = f dec ( z qt ; θ dec ) of an isotropic Gaussian distribution p ( s | z qt ) withσ 2 = 1 , where f dec ( · ; θ dec ) is the decoder of the VQ-VAE.
Following the conventional VQ-VAE training, the hash function is updated according to the gradient
[Equation]
where sg stands for stopgradient operator, T is an unroll length of the agent, and θ vq
= { θ enc , θ dec , θ e } .
3.4 Task-Specific Intrinsic Modulation
In a sparse reward environment, when an extrinsic reward signal has little or no presence, having atask-agnostic intrinsic motivation can be significantly beneficial in guiding the agent to efficientlyexplore the environment until the discovery of more extrinsic signals.
However, a task-generalintrinsic reward such as the inverse of the episodic count may become a distracting signal in the laterexploitation since it is not reduced with the progression of learning.
We thereby propose to includetask-specific modulation that aims to ultimately maximize the extrinsic return in the intrinsic rewardformulation of LECO.
The task-specific modulation can be seen as a dynamic factor for controllingthe exploration and exploitation, based on the extrinsic rewards earned from the environment.
For this, we leverage the bi-level optimization proposed in LIRPG [38].
First, let policy π parameterized by θ be updated using the policy gradient on the sum of intrinsic and extrinsic rewards,
[-E θ [ (cid:80) ∞ t =
-][Equation] where η is the step size.
We then construct our task-specific modulator for the intrinsic reward in Equation 1 as a parametric model such that r it =
(1 − λ ) r ep t + λr ta t ( a t − 1 , s t , a t ; θ ta ) where θ ta is themodulator parameters.
Now, the task-specific modulator is trained to maximize only the extrinsicrewards using the corresponding policy gradient such that
[Equation]
[-[ (cid:80) ∞ t =
-]
where η ta is the step size for updating θ ta . Here, it is noted that the expectation is performed on theupdated parameters θ ′ that is affected by our intrinsic reward and accordingly θ ta .
Therefore, θ ta isupdated by the chain rule such that ∇ θ ta J e = ∇ θ ′
J e ∇ θ ta θ ′ .
This can be considered a well-knownbi-level optimization framework popularly used in meta-learning, where the inner-loop and theouter-loop are corresponding to Equation 3 and Equation 4, respectively.
Model for task-specific modulation.
After executing an action a t , a task-specific modulationr ta t ∈ ( − 1 , 1) is modeled as
[Equation]
where 1 a is the one-hot vector for action a , f ta : R D
→ R |A| is a MLP network, and z et is sharedwith VQ-VAE based encoding to make the modulator efficient in training. We choose tanh , whichgenerates both positive and negative modulation, so that even in the absence of meaningful signalsfrom episodic counts, the modulator can act according to LIRPG and converge to a value thatmaximizes the extrinsic objective.
Experiments
The key concept of LECO is the learnable and modulated episodic count.
In other words, comparedto a naive episodic count that assigns the state novelty proportional to its visitation count withinan episode, LECO aims to produce low intrinsic rewards to task-irrelevant states regardless of itsepisodic count.
We demonstrate such behavior by comparing the mean intrinsic rewards obtained by
LECO to those from exploration with just the episodic count-based intrinsic reward in Figure 2.
Theresults show that while the episodic novelty remains high, modulation in LECO decreases the intrinsicreward such that the explorative behavior has vanished towards the end of training and the agentfocuses on the task-specific exploitation.
Apart from the novelty based on the agent’s location, in anenvironments where agent can pickup and drop objects, such actions can create task-irrelevant novelstates, further complicating the search space.
The task-specific modulation of LECO can mitigatethose task-irrelevant behaviors as shown in Figure 9 and Figure 11 in Appendix.
Environments.
As LECO operates on multiple components, we design the experiments to thoroughly cover various aspects, especially the adaptiveness and the scalability.
Since the real worldas an environment is nonstationary and requires high-dimensional perception, it is crucial for anagent to efficiently explore the constantly changing environment, even at a large scale.
Thus, wefirst demonstrate the explorative performance of LECO in procedurally generated environments of
MiniGrid [7].
Then, we further extend the experiments to large scale environments of DMLab tasks.
Baselines.
In order to present the contribution of each component of our proposed method, wecompare LECO to its variants that incrementally add each component to a baseline without theintrinsic motivation which we denote as no-Int for the rest of the section.
Similarly, a variant withonly the vector quantization episodic counts is denoted as VQ-only .
Additionally, a variant thatuses AE-LSH [31] as the hash function is denoted as LECO(AE-LSH) .
The no-Int baseline is an
IMPALA-based agent [12], with minor architectural modifications according to the environments.
Details on the base model and the architectures can be found in Appendix A. Other baselines include
DSC (Down Sampled Cell)
[10], AE-LSH [31], NovelD [36], and AGAC [13], where DSC uses anon-learnable hash for state compression and AE-LSH uses a learnable hash.
NovelD and AGACare state-of-the-art exploration methods for procedurally generated environments.
Here, for a fairassessment of LECO, we implement the episodic count of NovelD and AGAC using the VQ hashingas done in LECO for both of MiniGrid and DMLab tasks.
Experimental Setup.
In MiniGrid, LECO was trained using two A100 GPUs with a batch size of768 for 18 hours.
In DMLab, we used eight V100 GPUs with a batch size of 576 for 8 hours.
Theunroll length was T = 96 for all tasks and same LSTM-based policy network architecture was usedfor LECO and all other baselines.
Details on hyperparameters, model architectures, and trainingsettings are provided in Appendix B.
4.1 MiniGrid
MiniGrid [7] is a procedurally generated gridworld environment specifically designed to testand develop exploration algorithms under sparse rewards and partial observations.
A typicalworld in MiniGrid consists of M × N tiles, where each tile is assigned to be objects such asfloor, wall, lava, door, key, ball, box, and goal.
The agent deployed navigates through theworld, collecting objects and solving sub-tasks such as opening a door using a key, to reach [Figure] a goal placed somewhere in the world.
Of the many types of environments included, we testthe performance of LECO in three types of MultiRoom, KeyCorridor, and ObstructedMaze environments.
More specifically, we use total of six configurations of those environments namely MultiRoom-N6(MRN6) , KeyCorridorS4R3(KCS4R3) , KeyCorridorS6R3(KCS6R3) ,
ObstructedMaze-1Q(OM1Q) , ObstructedMaze-2Q(OM2Q) , and ObstructedMazeFull(OMFull) , roughly ordered in increasing level of difficulty.
The choices were made toreflect a wide range of difficulty including the most difficult task, while evaluating on similar tasksused by the previous baselines.
We emphasize that for all tasks the baselines and LECO only use the original partial input observations,in which a 7x7x3 tensor given as the partially-observable grid cell is the input observation for boththe policy network and intrinsic module as in [6].
This setting is more challenging than the othersettings such as the partial observation for the policy and full observation for the intrinsic module asin [26, 36, 28], and the partial observation for the policy and meta information such as the agent’sposition for the intrinsic module as in [13].
We first compare LECO to its variations as shown in Figure 3.
The variations include no-Int, DSC,
AE-LSH, VQ-only, and LECO(AE-LSH).
The results show that LECO solved all six tasks while VQ-only solved three, MRN6 , OM1Q , OM2Q , and show slow learning trend in OMFull .
On the otherhand, compared to DSC, learnable hashing methods for using episodic count as an intrinsic rewardbut with heuristic quantization, VQ-only outperforms DSC as DSC solved only one out of the sixtasks. While AE-LSH solved the same number of tasks as VQ-only, the scores of VQ-only are higherin all tasks. Without the task-specific modulators, the episodic count-only methods such as VQ-only and AE-LSH,tend to perform worse in KeyCorridor tasks than in ObstructedMaze tasks due to the task-irrelevantstate novelties.
More concretely, in OM1Q and OM2Q , the locked doors are generally located near thekeys making it easier for the agent to discover the sequence of opening the locked door by exploration,while in KeyCorridor, even though the map is more simple, the doors are generally placed furtheraway from the keys and thus increased search depth hinders the episodic count-only methods fromopening the locked door and eventually solving tasks. We visualize the agent’s behavior with andwithout the modulation in subsection D.1 of Appendix which depicts the episodic count-only agentchoosing task-irrelevant actions along the way in KeyCorridor task. As shown in Figure 3, LECO(AE-LSH) generally performs better than AE-LSH except OM1Q and OM2Q .
However, the performances of LECO(AE-LSH) are lower than those of LECO on all tasks, [Figure] which shows the benefits from VQ-VAE for the episodic counts. As shown by the results of no-Int, apolicy without any intrinsic reward is unable to solve any of the six tasks.
Combining all comparisons gives indications that 1) episodic count enables training in sparse rewardexploration tasks with procedurally generated environments, 2) vector quantization-based counting ismore effective than heuristic cell discretization (DSC) as well as AE-LSH, and 3) the task-specificmodulation of LECO further improves the performances of the episodic count-based approach.
Moreover, we compare the performance of LECO with other baselines of AE-LSH, AGAC, and
NovelD on the same six MiniGrid tasks.
As shown in Figure 4, only LECO solves all six tasksincluding OMFull .
Here, it is noted that the original implementations of AGAC and NovelD usedthe agent’s position and the full observations, respectively, for the episodic counts in their methodswhile we used the original partial observations in our implementations of AGAC and NovelD. To thebest of our knowledge, LECO is the first to achieve average score of over 0 .
75 on the OMFull withcompletely partial observation setting within 300M steps. NovelD and AGAC have shown to solve
OMFull in their papers.
However, we observe that when we modify their official codes to utilize thepartial observations in obtaining the episodic counts their performances substantially dropped as inour results. Recently, MADE [35] has also shown to solve OMFull , however, detailed descriptionson their implementation and hyperparameters are missing and the official code does not supportexperiments on MiniGrid. We conclude that MADE cannot be fairly compared without the details ontheir MiniGrid experiments.
In MultiRoom and KeyCorridor environments, the increasing rates inscores of LECO is slightly slower than those of NovelD since the modulator in LECO requires a bitmore experiences to train, however, the advantage of having the modulator is clearly shown throughsignificant performance improvement in the harder exploration tasks. In addition, LECO is morestable in the later phase of KeyCorridor training.
4.2 DMLab
DMLab [3] is a learning environment consisting of various tasks, each involving different visuals andobjectives in a simulated 3D world.
We consider DMLab for its partial-observability that naturallyarises from first-person perspective and its complexity from image-based observation with fine detailsand randomly generated levels to demonstrate the scalability of LECO. We compare LECO and
LECO(AE-LSH) to no-Int, episodic count methods based on AE-LSH, DSC, and VQ-only, andstate-of-the-art exploration methods including NovelD, RND [5], and RIDE [26] on two individualexploration-heavy tasks.
For RND and RIDE, the architectures of neural networks are based on the RND network used in NovelD for MiniGrid and the forward/inverse networks used in RIDE for
[Figure]
MiniGrid 1 . We use the same RND networks for RND and NovelD and the same architecture of VQfor LECO, NovelD, and RIDE in their episodic count module. We select two specific tasks from DMLab, lasertag_three_opponents_small andlasertag_three_opponents_large , in order to show LECO’s capability to solve exploration tasks at a large scale. In both tasks, an agent is deployed in a procedurally generated 3-Denvironment where it has to search and shoot three opponents moving around the maze-like map(see Figures 15 to 17 for example screenshots).
Since the reward is only given when the agentsuccessfully tags the opponents by shooting a laser, these tasks are known to be difficult to solvewithout an advanced exploration strategy and are characterized as sparse reward problems.
Thesmall and large stand for the size of the map, which determines the reward sparsity.
In Figure 5, we present the learning curves of LECO and baselines on these tasks.
Each task has itsown independent agent, but the architectures and hyperparameters remain consistent across thesetasks.
To the best of our knowledge, experiments for RND, RIDE, and NovelD on DMLab hasnot been published so far. As shown in Figure 5, these models show decent performances onlasertag_three_opponents_small , but perform worse than LECO and moreover do notsolve the task of lasertag_three_opponents_large at all.
Conclusion
This paper proposes a learnable episodic count to produce task-specific intrinsic rewards for hardexploration problems including high-dimensional state spaces and procedurally generated environments.
In specific, the hash code based on VQ-VAE is used for computing the episodic state countand the corresponding state novelty, and this episodic novelty is regulated by the task-specific modulator that is trained to maximize the extrinsic rewards.
Experimental results demonstrate that theproposed task-specific exploration based on the episodic count significantly outperforms the previousstate-of-the-art exploration methods on the most difficult tasks of MiniGrid and DMLab.
However,the proposed intrinsic reward also has a number of limitations. First limitation comes from solvingtasks that require intensive leveraging of the inter-episode novelty. For example, objects of negativereward could be common among different episodes, and in that case, the inter-episode novelty canlead an agent to avoid those objects. Second limitation is dealing with somewhat difficult bi-leveloptimization.
For future works, we will extend our work to combine the inter-episode novelty anddevise a more efficient training of task-specific intrinsic reward to further improve the scalability.
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A Architecture Details
We employ two modules, the policy module and the intrinsic module.
The overall network architectures for these modules are described in Figure 6 and their details are presented in Table 1 and
Table 2.
The base architectures of the policy network for MiniGrid and DMLab are based on that of
AGAC and IMPALA, respectively.
One difference is that the value head of the policy module, whichoriginally outputs the value estimation of extrinsic rewards V e , estimates the value function for thesum of extrinsic and intrinsic rewards, denoted V e + i .
The value estimation for extrinsic rewards isnow done in the intrinsic module, as shown in the right hand side of Figure 6.
B Hyperparameters and Training Details
All baseline models as well as LECO are trained using the V-trace actor-critic framework, IMPALA[12], where the policy and the value networks are learned from trajectories collected from multipleactors running asynchronously in a distributed system.
The total number of actors used in MiniGridtasks and DMLab tasks are 20 and 240, respectively, spread across multiple nodes.
Table 3 presentsour common hyperparameter values for all models on MiniGrid and DMLab tasks.
It is noted that the
ObstructedMaze tasks in MiniGrid used γ = 0 .
8 while all other tasks, including the DMLab tasks,used γ = 0 .
We observe that ObstructedMaze tasks, especially the ObstructedMaze-Full is solvedwith a lower γ value.
For RND, NovelD, and RIDE on DMLab tasks, we search over learning-rate ∈{ 0 . 0001 , 0 . 0002 , 0 . 0004 } , and the intrinsic reward coefficient ∈ { 0 . 1 , 0 . 01 , 0 . 005 } and select thelearning rate as 0 . 0001 and the intrinsic reward coefficient as 0 . 01 .
[Table]
[Table]
[Table]
[Table]
[Table]
ConvBlock
[Table]
[Table]
(4, 2), ReLU, AvgPool, Conv2d( 1 × 1 , 64 , stride=1)]
×
ConvBlock(4, 2)
Residual Block ×
VQ-Decoder
Symmetric architecture of Encoder
Modulator
FC(576, 512)
FC(384, 512)
ReLU
FC(512 + |A| , |A| )
tanh
Value Head
FC(576, 512)
FC(384, 512)
ReLU
FC(512, 1)
C Ablation Study
C.1 Hash size.
In our VQ-based hashing, used in LECO and the baselines, the hash size is defined as the spatial sizew × h of the output of the encoder of VQ-VAE.
Accordingly, the capacity of the hash is defined as
K w × h where K is the codebook size.
This capacity can be critical in measuring the state noveltybased on the episodic count, as it determines the level of the state compression and the corresponding
[Table]
[Table]
state clustering.
If the capacity is too large then most states would be mapped to different hashes andif it is too small then most states would be mapped to the same hash.
In MiniGrid, we search over the spatial size ∈ { 2 × 2 , 3 × 3 , 4 × 4 } and the codebook size ∈ { 8 , 16 } .
In DMLab, we search over the spatial size ∈ { 3 × 2 , 6 × 4 } and the codebook size ∈ { 12 , 24 , 36 } .
The key results of this grid search for LECO are shown in Figure 7.
As shows in the figure, 3 × 3 × 8performs the best in MiniGrid tasks and 3 × 2 × 24 performs the best in the DMLab tasks and thusthe two parameter values are selected as the default values for our hashing as in Table 4.
For AE-LSH, we search over the hash size ∈ { 9 , 15 , 25 , 35 } in MiniGrid, and ∈ { 24 , 42 , 62 , 82 } in
DMLab and select 25 and 62 as the hash size of each task.
For a given state, DSC hash is definedas follows: 1) downsample it to w × h by nearest neighbor algorithm, 2) rescale the intensity sothat they are integers between 0 and K for each channel of the state, and 3) serialize it.
For DSC in
MiniGrid, we search over the downsampled size ∈ { 2 × 2 , 3 × 3 , 4 × 4 } , and in DMLab we searchover the downsampled size ∈ { 3 × 2 , 6 × 4 } and the intensity ∈ { 4 , 8 } , and select 3 × 3 × 11 and3 × 2 × 4 as the hash size for MiniGrid and DMLab, respectively.
C.2 Study of λ .
Equation 1 indicates the possibility of adjusting the weight to the task-specific modulation.
In orderto show the effect of this weight, we run an ablation on Equation 1 where the equation is restatedhere for convenience:
[Equation]
[Figure]
We fix λ as a constant hyperparameter and its value is determined by grid search on three MiniGridtasks as shown in Figure 8 of Appendix.
On the other hand, lambda can be adaptively calculatedrelative to the magnitude of the novelty from episodic counts, however, we leave it as the future work.
When λ = 0 , the algorithm is equivalent to using episodic count only, which is denoted as VQ-onlyin the main text.
When λ
= 1 . 0 , the algorithm is similar to using LIRPG [38] only, however, note thatthe architecture of the task-specific modulator are fairly different from the intrinsic module in LIRPG,and the final optimization loss of the intrinsic module in LECO is also different in that it includesa VQ-VAE loss as a part through the shared encoder.
When λ = 0 .
5 , it is equivalent to LECO.
Asshown in Figure 8, having only either the episodic state novelty or the task-specific modulation resultsin failure in solving the task.
On the other hand, putting more weight on the episodic state novelty( λ = 0 . 25 ) relatively performs better than small weight ( λ = 0 . 75 ), however the performance is bestwhen the two terms are balanced ( λ = 0 . 5 ).
D Discussions
D.1 Task-irrelevant actions in count-only methods.
In order to solve the KC tasks, an agent is required to obtain a key, open a locked door, and obtainthe target object, which is a colored ball.
Without task-specific modulation the agent can easily getstuck on repeating meaningless actions such as dropping the key or opening and closing the door,to obtain task-irrelevant state novelties.
Figure 9 visually depicts such phenomenon.
In this figurewe provide action heat maps on three distinct episodes.
Each map displays the accumulated countson three specific actions ( pickup, drop, toggle ) from different episodes which are sampled by thefinal checkpoint of each model.
Red colored grids represent the agent’s execution of specific actionfor more than once in that position.
For example, in episode-1 of the top-row, the agent performsthe specific action twice below of the gray key.
In top-row of the figure we can observe the agent of
VQ-only performing the toggle action multiple times in front of the doors and the pickup & dropaction repeatedly in empty spaces.
Moreover, the agent opens the door to the room with the targetobject but does not proceed to picking it up (see Episode-3 ).
On the other hand, the bottom row of
[Figure]
the same figure shows that LECO focuses on achieving the task objective by faithfully conducting
-related actions.
D.2 Task-specific modulation in r i .
The task-specific modulation of LECO is designed to dynamically control exploration and exploitationduring training.
Here, we conceptually illustrate the trends of LECO’s intrinsic reward r i as thelearning progresses.
In the beginning of training, before collecting enough extrinsic rewards, the agentwould not have any information about task-relevant and -irrelevant behaviors.
In hard explorationproblems, the agent cannot obtain extrinsic rewards without utilizing the state novelty (i.e. exploration)in the environment.
Assuming that exploration by episodic state novelty is enough to guide theagent to achieve some extrinsic rewards, those extrinsic rewards would cause r ta to be positive toencourage the agent to explore more in that direction.
As the agent learns explorative behaviorsand collects some extrinsic rewards, task-specific modulation will start to distinguish task-relevantfrom the irrelevant amongst those learned behaviors.
Then, the agent transitions to focusing on thetask-relevant behaviors and eventually ignore the task-irrelevant behaviors by decaying the intrinsicreward as a whole.
The results in Figure 12 depicts the described decaying trend.
We can also observethat the dynamics of LECO’s intrinsic reward r
i is different for each task; r i decays more moderatelyas the relative difficulty and extrinsic reward sparsity increases.
The difficulty and sparsity roughlyincreases from (a) → (c), (d) →
(f) and from (g) → (h).
D.3 Comparisons of hashing results.
First, We qualitatively show some weaknesses of AE-LSH compared to VQ.
In general, both hashmethods map similar states into the same hash, however, some differences can be observed in theobtained hashing results.
As shown in Figure 13 and Figure 14, in Minigrid, AE-LSH maps importantstates (i.e. states in which the agent realizes a target object or states just before obtaining a targetobject) and relatively less important states into the same hash.
This obstructs correctly distinguishingtask-specific state novelty for efficient exploration.
On the other hand, VQ more clearly clusters statesto the hash according to the relative importance.
In DMLab-lasertags, the difference in performance between AE-LSH and VQ is also reflected in thehashing results.
As shown in Figure 15, Figure 16, and Figure 17, AE-LSH maps states to hashesaccording to visual similarities whereas VQ learns to map according to the importance representedvia the presence of the opponents in sight.
[Table]
We quantitatively show the effectiveness of VQ compared to AE-LSH.
As shown in Table 5, on the
DMLab-Lasertags, the new hash rates, which measure how often the states are mapped into a newhash within an episode, obtained by VQ are smaller than those by AE-LSH even though the entirehash space of VQ is larger than AE-LSH (i.e. 6 24 > 2 62 ).
This means that VQ makes similar statesto be more grouped into the same hash.
D.4 Noisy-TV on MiniGrid.
Provided by [26] is a variation of MultiRoom task that brings the concept of Noisy-TV probleminto the MiniGrid environment setting, named MiniGrid-MultiRoomNoisyTV-N7-S4 .
In thisversion of the task, the color of the ball changes to a random color when the agent performs a specificaction, creating a visual novelty irrelevant to the objective of the task, which is to reach the goal [26].
In Figure 10, we present the learning curves of LECO and baselines on this Noisy-TV environment.
The result shows that all methods that use the episodic intrinsic reward, including LECO, are able tosuccessfully solve the noisy environments.
In this experiment, We conjecture that the task-specificmodulator in LECO requires a little more experiences than NovelD since it is updated upon theextrinsic rewards that are sparse in the early RL phase.
In a similar argument, DSC, which requires no training, is most efficient in the noisy TV problem.
Moreover, in fact NovelD converges stably inabout 3e7 frames (vs. 5e7 frames for LECO).
[Figure]