Unsupervised Skill Discovery via Recurrent Skill Training
Anonymous Author(s)
Abstract
Being able to discover diverse useful skills without external reward functions isbeneficial in reinforcement learning research.
Previous unsupervised skill discoveryapproaches mainly train different skills in parallel.
Although impressive resultshave been provided, we found that such parallel training procedure inherently discourages exploration, which leads to poor state coverage and restricts the diversityof learned skills.
In this paper, we take a deeper look into this phenomenon andpropose a novel framework to address this issue, which we call Recurrent Skill Training (ReST).
Instead of training all the skills in parallel, ReST trains differentskills one after another recurrently, along with a state coverage based intrinsicreward.
We conduct experiments on a number of challenging 2D navigation environments and robotic locomotion environments.
Evaluation results show that ourproposed approach outperforms previous parallel training approaches in terms ofstate coverage and skill diversity.
Videos of the discovered skills are available athttps://sites.google.com/view/neurips22-rest .
Introduction
[Figure]
Recent advances in deep reinforcement learning have shown its promising performance indomains ranging from game playing [2, 3],robotics [4, 5] and recommender systems [6].
These applications of reinforcement learningrely on task-specific reward functions for theagents to successfully accomplish the tasks.
However, intelligent creatures can automaticallyexplore the environments and learn diverse useful skills in the absence of external supervision.
Such ability is beneficial in a variety of situations.
For tasks where rewards are non-trivialto design or where the reward signal is sparse,unsupervised skill discovery approaches can provide intrinsic rewards to help accomplish tasks.
Moreover, in hierarchical control problems, unsupervised skill discovery can serve as low-levelpolicies for downstream tasks [7, 1].
Although existing works have shown great potential in discovering diverse useful skills in an unsupervised manner [1, 7, 8], one of the key problems of such methods, as observed in our preliminaryexperiments and some recent works [9, 10], is that they might suffer from poor state coverage, which may lead to failures in learning desirable useful skills.
For instance, in robotic locomotionenvironments, previous approaches tend to learn ‘posing’ skills instead of dynamic, far-reachingskills [1].
A straightforward explanation for this phenomenon would be the lack of exploration [9].
However, we argue that this is not the only reason for the poor state coverage.
Counterintuitively, weobserved that the skills after convergence may avoid visiting certain states even if they are exploredduring training .
For instance, as shown in our preliminary 2D navigation experiments, the discoveredskills fail to cover the states passing through the bottleneck to the right (Figure 1b), even if they havebeen explored during training (Figure 1a).
We call this phenomenon Exploration degradation .
In this paper, we take a deeper look into the above phenomenon and show that it is mainly causedby the parallel training paradigm, which is a common choice for most existing works [1, 7, 8, 10].
When multiple skills trained in parallel have visited the same state, such state will be preventedfrom being visited again.
Detailed analysis is provided in Section 3.1.
Based on the analysis, wepropose Re current S kill T raining (ReST), an unsupervised skill discovery algorithm that addressesthe exploration degradation issue.
Instead of training all the skills in parallel, ReST trains the skillsone after another in a recurrent fashion, along with an intrinsic reward that discourages coveringfrequently visited states of other skills.
A preliminary result of using ReST is shown in Figure 1c,where the exploration degradation problem is eliminated and the converged skills are able to visit thespace in the right room.
Evaluation results on complex 2D navigation and robot locomotion tasksshow that our approach can achieve better state coverage and skill divergence compared to baselines.
Our contributions are summarized as follows:
• We discover a new phenomenon that reduces state coverage called exploration degradation,which indicates that some certain states are discouraged from being visited by the learned
skills, even if they have been explored during training. • We show that the main reason causing exploration degradation is that multiple skills visiting the same states can reduce the MI reward in the parallel training paradigm.
We then proposeRe current S kill T raining (ReST), a recurrent training paradigm along with a state coveragebased intrinsic reward, which prevents multiple skills from visiting the same states andalleviates the exploration degradation issue.•
We conduct experiments on various 2D navigation tasks and robot locomotion tasks.
Evaluation results show that our method achieves better state coverage and divergence comparedto baseline methods.
Moreover, ReST learns diverse meaningful robot locomotion skillsthat have not been shown in previous works.
Preliminaries2.1 Markov Decision Process
Markov decision process (MDP) can be used to find a reward maximizing policy.
It is representedby a tuple
( S , A , P , R, γ, µ ) , where S , A are the state and action spaces.
P : S ×
A × S → [0 , 1]is the transition dynamic that maps the state and action into a probability distribution over the nextstate.
R : S ×
A → R is the reward function.
γ is the discount factor for the reward function whileµ : S → [0 , 1] is the initial state distribution.
The expected discounted cumulative reward can beformulated as J R
[-= E ⌧ ⇠ ⇡ [ P 1 t =
-][Equation]
t , [Equation] t )] .
Thus the overall optimization problem can bewritten as [Equation]
2.2 Unsupervised Skill Discovery
Generally speaking, unsupervised skill discovery aims to find a family of skills conditioned on latentz , which results in a latent-conditioned policy π ( a | s, z ) that maximizes the mutual informationbetween state and latent:
[Equation]
where s ∈ S , a ∈ A , z ∈ Z are the state, action and latent respectively.
Let ( S, Z ) ∼ p
( s, z ) asthe random variables of the state distribution and the latent distribution.
As suggested by [11], avariational lower bound for Equation (2) can be derived as:
[Equation]
where q ϕ ( z | s ) is a learned discriminator approximating p ( z | s ) .
Such lower bound also exists for
Equation (3):
[Equation]
where q ϕ ( s | z ) is a learned function approximator of p ( s | z ) .
Recurrent Skill Training
[Figure]
[Figure]
[Figure]
(c)
[Figure]
In this section, we first introduce the exploration degradation phenomenon of previous skill discoveryapproaches with parallel training paradigm.
Then we present our proposed recurrent skill trainingmethod that addresses this issue.
[Figure]
Initialize: random initialized skill π , RND networks ˆ f and f for i = 1 to N do
Set skill π i = π
Set RND network ˆ f i
=
ˆ f , f i = f
Collect on-policy samples with π i
Update RND network
ˆ f i by minimizing loss in Equation (8) end for repeatfor i = 1 to N do for j = 1 to M do
Collect on-policy samples with π i ;
Calculate reward using Equation (10);
Update skill policy π i using any RL algorithms;
Update RND network
ˆ f i by minimizing loss in Equation (8) using the latest on-policy samples; end for end for until convergence
3.1 Exploration Degradation
As we observed in our preliminary experiment shown in Figure 1, previous unsupervised skilldiscovery approaches might suffer from the exploration degradation problem, such that some exploredstates (e.g, states near the bottleneck and in the right room in Figure 1a) are prevented from beingvisited by the learned skills.
We now provide a simplified analysis to show that this phenomenon ismainly caused by the parallel training paradigm commonly used in previous works.
Let us focus on discrete latent case with N different skills.
Consider at state s , skill k visited state swith p ( z k | s ) probability, which is modeled by the discriminator q ϕ ( z k | s ) .
Assume state s 0 is onlyvisited by skill k
whereas s 1 is visited by multiple skills, which means p ( z k | s 0 )
= 1 and p ( z k
| s 1 ) < 1 .
As long as the discriminator q ϕ can model the difference and output q ϕ ( z k | s 0 ) >
q ϕ ( z k | s 1 ) , which isnot a strong assumption, the intrinsic reward used by MI approaches would encourage the visitationof state s 0 by skill k and discourage visiting s 1 .
This means MI based approaches end up withskills that prefer exploiting states only visited by themselves in the previous training epochs, causingexploration degradation.
For clarity, we further explain this issue with a toy example.
Consider a 2 × 2 grid world as shownin Figure 2.
There are four states 0 , 1 , 2 , 3 and at each state, there are three actions to take: go tothe two adjacent states or stay where it is.
For simplicity, we use the number of the next state todenote the action, for instance, if the agent is in state 0 and chooses to go to state 1 , then the actionwill be denoted as 1 .
We use the mutual information described in Equation (2) and assume that thediscriminator is perfect: q ϕ ( z | s ) =
p π ( z | s ) .
Consider a case when the number of skills is N = 2and during the first collection of samples, skill 1 visited { 0 , 2 , 3 } while skill 2 visited { 0 , 1 , 3 } , asshown in Figure 2a.
Since we have a perfect discriminator, for state 3 the discriminator wouldoutput q ϕ ( z 1 | 3)
= q ϕ ( z 2 | 3) = 0 . 5 , which results in 0 reward for both skills.
Therefore, the optimalconverged policy for skill 1 would generate trajectory { 0 , 2 , 2 } while skill 2 would generate trajectory{ 0 , 1 , 1 } , as shown in Figure 2b.
State 3 is not covered, which is undesirable.
This example indicatesthat even in such an extremely simple case, the exploration degradation phenomenon still exists.
3.2 Recurrent Skill Training
Instead of training all skills in parallel, we propose a recurrent training paradigm, along with a statecoverage intrinsic reward.
We now introduce the details of our proposed method.
Recurrent Training Paradigm.
As analyzed in Section 3.1, the main reason causing the explorationdegradation issue is that the same states are visited by multiple skills in the parallel training paradigm.
A natural way to alleviate this issue is to train the skills one after another recurrently.
In this way, we can encourage the latter trained skills to avoid entering the same states covered by the previous skills.
Figure 3a illustrates the recurrent training paradigm compared with parallel training.
Starting with Nrandomly initialized skills ( N = 3 in this case), the recurrent training paradigm trains one skill ateach epoch whereas the parallel training paradigm trains all the skills together.
Furthermore, in orderto improve convergence, the recurrent training paradigm updates each skill for M epochs ( M = 2 inthis case) before switching to another skill.
In this work, we use N independent neural networks toparametrize the N skills, which can be considered as discrete latent conditioned policies.
State Coverage Intrinsic Reward.
When implementing the above recurrent training paradigm,the latter trained skill needs to avoid visiting the states frequently visited by other skills.
In orderto accomplish this objective, we need to identify how frequently each state is visited by each skill,or equally, how novel a given state is to a skill.
In this paper, we adopt random network distillation(RND)
[12], a simple yet scalable novelty detection approach, to estimate the novelty of a state to aspecific skill.
For each skill, we have two neural networks for state novelty detection: a randomly initialized fixedtarget network and a prediction network. During training, the fixed target network is set as the groundtruth while the prediction network is trained to fit the target network’s output.
The input data iscomposed of the states visited by this skill, which follows the state distribution s ∼ p ( s | z i ) .
Wedenote the target network for skill with latent z i ∈ Z as f i : S → R k and the prediction network asˆ f i : S → R k , where k is the dimension of the two networks’ output and i ∈ { 1 , 2 , ..., N } is the indexof the skill. The prediction network ˆ f i is optimized by gradient descent to minimize the expectedmean square error loss L i :
[Equation]
As suggested by [12], with the learned prediction network, we can estimate the frequency of state svisited by the i th skill using the prediction error
||
ˆ f i ( s ) − f i ( s ) || 2 .
Intuitively, a higher predictionerror indicates higher uncertainty of ˆ f i on this state, which further implies its higher novelty.
Sincewe need to avoid visiting states visited by other skills when training a certain skill, a straightforwardsolution is to define a reward function for skill i with state s t and action a t as:
[Equation]
such that states frequently visited by any other skills are less desired.
However, this minimumoperator would make the reward landscape rugged, which might lead to poor convergence property.
To stabilize the training process, we introduce a soft version of the minimum operator in (9):
[Equation]
where α is a task-specific temperature parameter.
Practical Algorithm.
We summarize our algorithm in Figure 3b and Algorithm 1.
Firstly, werandomly initialize a policy network π i and a pair of RND networks f i and ˆ f i for each skill i ∈{ 1 , 2 , ..., N } .
Before training skills, ReST first collects on-policy samples for each initialized skilland train their RND networks.
Then we train each skill’s policy network and the corresponding RNDnetworks for M times during each training epoch.
The skill networks are trained recurrently untilconvergence.
For maximizing the intrinsic reward, the ReST algorithm can be combined with anarbitrary reinforcement learning algorithm.
In this paper we choose Proximal Policy Optimization(PPO) [13] with generalized advantage estimation (GAE) [14].
Grid World Example.
We further illustrate the effectiveness of the proposed algorithm using the2 × 2 grid world example with the same setting as in Section 3.1.
As suggested by our previousanalysis, the parallel training paradigm will end up with two skills that neither of them visits state 3 .
As mentioned in Section 3.1, the initial two skills visits { 0 , 2 , 3 } and { 0 , 1 , 3 } respectively.
Here weassume that the RND networks f i and ˆ f i perfectly obtain the state visitation frequency, which means:
[Equation]
if state s is visited by skill i r otherwise (11) where r > 0 is the prediction error.
ReST starts by training the corresponding RND networks of eachskill and then recurrently train different skills to maximize the intrinsic reward.
For skill 1 , visitingstate 2 would gain reward r while visiting state 1 and 3 would get zero rewards.
Therefore, theoptimal policy for skill 1 would be visiting { 0 , 2 , 2 } in sequence.
After updating the RND networksof skill 1 correspondingly, for skill 2, visiting state 1 and 3 would gain reward r while visiting 2would get zero rewards.
Therefore, the initial skill 2 is already the optimal policy and the trainingconverges.
This way, the optimal trajectory for skill 1 is { 0 , 2 , 2 } while for skill 2 it is { 0 , 1 , 3 } , asshown in Figure 2c.
Therefore, the state space of the 2 × 2 grid world is fully covered by the twoskills.
Experiments
[Figure]
be found in the Appendix We conduct experiments on several challenging 2D navigation environments and several roboticlocomotion tasks.
We compare our proposed approach with two of the most popular unsupervisedskill discovery approaches, DIAYN [1] and DADS [7].
Both of these approaches use the paralleltraining paradigm to optimize different skills.
Since our proposed approach parameterizes each skillwith an independent neural network, we also compare our proposed approach with DIAYN and DADSusing independent neural networks, which we call DIAYN-i and DADS-i respectively, for a faircomparison.
We present our empirical results both qualitatively and quantitatively.
The quantitativeresults introduce a state coverage metric and a mutual information metric.
Our proposed approach
[Figure]
[Figure]
Generally speaking, baseline methods discover ‘posing’ skills instead of dynamic locomotion skills whereas ReST discovers dynamic, far-reaching locomotion skills
[Figure]
significantly outperforms previous skill discovery approaches in terms of state coverage while stayingcomparable with previous approaches in terms of mutual information.
Qualitative results includevisualizations of the converged navigation experiments, the state visitation of robotic locomotiontasks, and rendered videos of novel skills discovered using our proposed approach, which can befound in our project website https://sites.google.com/view/neurips22-rest .
Details ofour implementation can be found in the Appendix.
4.1 2D Navigation Tasks
Environments.
We conducted experiments on several challenging 2D navigation environments.
Theagent is a point mass navigating in a 2D plane with boundaries [0 , 1] ×
The observation spaceof the environment has 4 dimensions, including the x ∈ [0 , 1] and y ∈ [0 , 1] position on the plane and the corresponding velocity v x and v y belonging to the velocity space { ( v x , v y ) | v 2 x + v 2 y ≤ 0 . 01 } .
The action space has 2 dimensions, including the acceleration a x ∈ [0 , 0 . 1] and a y ∈ [0 , 0 . 1] of thepoint mass agent.
There are walls inside the plane and the agent cannot go through the walls.
Wedesigned diverse placements of the walls with increasing difficulties for exploring the environments,which could be used to test the effectiveness of our proposed approach.
Evaluation Metrics.
We introduce two metrics to compare our proposed approach with our comparison baselines.
The first one is the state coverage metric.
State coverage matters since not coveringenough state space might result in failures in learning desirable useful skills.
We evaluate the statecoverage on the X - Y plane by first decomposing the environment into cells and then testing whethereach cell is successfully visited.
We roll out 1 trajectory for each skill and use the visited statesto calculate the state coverage metric.
The percentage of cells visited by at least one of the skillsis the state coverage.
Moreover, merely covering the state space is not enough.
The skills need tobe informative about which states they are going to visit in the environment so that the skills aremeaningful.
We use mutual information between skill latent z and the corresponding covered states sto quantify how informative the skill is.
Similarly, we decompose the state space into multiple cellsand roll out 20 trajectories for each skill to record the probability distribution of states p ( s ) for all thestates visited by the skills.
Then we record the probability distribution of states visited by each skillp ( s | z ) and calculate the entropy H ( S ) and H ( S | Z i ) .
Since the skill distribution p ( z ) is a uniformdistribution for all skills, we can calculate the mutual information as:
[Equation]
Qualitative Results.
We train N = 10 skills for each of the algorithms and rollout 20 trajectories toobtain the qualitative results.
We conduct experiments on 4 challenging 2D navigation mazes, whichwe call the DoorMaze , CenterMaze , 4RoomMaze and 9RoomMaze , which are shown in Figure 4.
Theleft 5 columns show the qualitative results.
Each rolled-out trajectory is rendered as a curved lineand different colors represent different skills.
The results indicate that our proposed ReST algorithmcan reach more diverse states in the environments whereas baseline approaches like DIAYN or
DADS can only reach a small portion of the environments.
Usually, the baseline approaches cannotpass through the ‘bottlenecks’ in the environment.
The results support our insight that the paralleltraining paradigm is one of the causes of previous skill discovery approaches’ state coverage issues.
Moreover, when using independent neural networks to parameterize different skills, we observe thatthe performance is not so different from the latent-conditioned parameterization version.
Quantitative Results.
Beyond qualitative results, we quantify the state coverage and the informativeness of different algorithms using the aforementioned two metrics.
As shown in the right two columnsof Figure 4, our proposed ReST algorithm significantly outperforms baseline approaches with aparallel training paradigm in terms of state coverage and is comparable with baseline approachesin terms of mutual information I ( S ; Z ) .
The above results indicate that our proposed approach cancover diverse sets of states without too much sacrifice on informativeness.
Quantitative results alsoevidenced that the difference in performance is not because of the different parameterization since
DIAYN and DADS using independent neural networks are not so different from the original ones.
4.2 Robotic Locomotion Tasks
We present qualitative results of robotic locomotion skills discovered using our proposed ReSTalgorithm in MuJoCo [15].
Generally speaking, our proposed ReST algorithm can discover dynamic,far-reaching robotic locomotion skills whereas DIAYN and DADS tend to discover ‘posing’ skills.
Figure 5 shows visualizations of parts of skills discovered using ReST.
There are several novel skillsdiscovered, such as Hopper backflip, that have not been presented in previous works to the bestof our knowledge.
More rendered results can be found in the Appendix and our project websitehttps://
Moreover, we provide semi-quantitative results of the proposed approach.
As shown in Figure 6, wedraw the agent’s x position over timestep, using the skills discovered by ReST and the comparisonbaselines.
The results are evaluated on HalfCheetah , Hopper and Walker2d tasks.
We use the
OpenAI Gym [16] settings of the three tasks, where HalfCheetah is trained with fixed episode length whereas Hopper and Walker2d terminate when the agents fall during training.
The resultingtimestep- x curve indicates that our proposed approach learns skills that are more far-reaching, diverse,and dynamic than baseline methods.
This also evidenced that our recurrent training paradigmoutperforms the parallel training-based baselines by alleviating their state coverage issues.
Related Work
Unsupervised Skill Discovery.
Previous unsupervised skill discovery approaches mainly focus onmaximizing the mutual information I ( S ; Z ) to obtain meaningful skills.
As discussed in Equation(2) and Equation (3), the mutual information has two forms.
Several works follow the Equation (2).
VIC [17] uses the mutual information between the final states and the skill latent as the intrinsicreward and optimizes it via reinforcement learning.
DIAYN [1] fixes the prior p ( z ) and uses themutual information between the skill and its visited states as its intrinsic reward to learn meaningfulskills, which has superior performance compared to VIC.
VALOR [8] further improves DIAYNby replacing the state-based objective with a trajectory-based objective.
DADS [7], on the otherhand, uses the Equation (3) form of mutual information and learns a transition model q ϕ ( s [-
-]′ | s, z )and uses model predictive control to solve downstream tasks.
EDL [9] provides insight into whyprevious approaches suffer from lack of exploration and proposes an algorithm using a fixed stateprior p ( s ) that to alleviate the issues.
IBOL [10] tries to relieve the difficulty of reaching diversestates by introducing a low-level controller.
Besides learning a set of skills, SMM [18] formulatesskill discovery as a state marginal matching problem and optimizes the KL divergence between theexpected state distribution and the current policy’s state distribution.
MUSIC [19] improves previousunsupervised skill discovery algorithms by adding the mutual information between the surroundingstate and the agent state.
DDL [20] learns one skill that maximizes the dynamical distance functionsof the previous skill.
Besides the above approaches, there are also other unsupervised skill discoverymethods [21, 22, 23, 24].
[Table] Reward.
Another stream of works related to this work is intrinsic reward.
In our proposedapproach, we use intrinsic reward as an objective to help learn a set of meaningful skills.
Intrinsicreward can also help with cases where rewards are sparse by augmenting them to the original rewardfunction.
Count-based exploration [25, 26, 27, 28] uses pseudo count to identify the frequentlyvisited states and the less frequently visited ones and adds the count-based bonus as intrinsic rewardsto accelerate exploration.
Prediction error exploration methods [29, 30, 12, 31, 32] make use ofprediction errors as intrinsic rewards based on an insight that states with high prediction error shouldhave higher novelty.
Other works augment an information-theoretic intrinsic reward with extrinsicrewards that encourage information gain about the environments [33, 34, 35].
Discussion
In this paper, we proposed a novel, effective yet simple algorithm called Recurrent Skill Training(ReST).
We began by introducing a new phenomenon called exploration degradation which reducesstate coverage of the learned skills.
We found the key reason for this phenomenon is the paralleltraining paradigm commonly used in previous skill discovery approaches, such that the same statesvisited by multiple skills are discouraged from being visited again.
Instead of training all skillsin parallel at each epoch, ReST trains different skills one after another recurrently.
This recurrenttraining paradigm is supported by an effective prediction error-based intrinsic reward inspired bynovelty detection methods.
We then conducted experiments on 2D maze navigation to continuousrobotic control tasks.
Both qualitative and quantitative results show that ReST is able to discover morediverse skills with better state coverage compared to baseline algorithms.
Moreover, we demonstratedseveral novel and dynamic robot locomotion skills that have not been presented in previous works.
There are also some limitations of the proposed algorithm.
First of all, compared with previousapproaches, our proposed approach has worse sample complexity during skill discovery since onlyone skill is trained at each epoch, as shown in Figure3a.
Moreover, the computational complexityis higher than approaches like [1] or [10] since ReST needs to compute intrinsic reward based onall other skills’ prediction errors.
Finally, due to the recurrent training paradigm, ReST is currentlynot scalable to continuous latent, which is in general a better choice as a low-level controller forhierarchical control in downstream tasks.
Future works include addressing the above limitations andapply ReST to downstream/hierarchical tasks.
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