Add trajectory transformer
Author: CarlCochet
Pull request: yes
Labels: name: New model

# What does this PR do?
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Add Trajectory Transformer to transformers.

This is a transformer model used for offline deep reinforcement learning, taking observations, actions and rewards as one big sequence. 
The model is adapted from the code [here](https://github.com/jannerm/trajectory-transformer). 
The caching mechanism as well as attentions outputs and hidden states outputs are additions not present in the original code. 

The adapted forward pass produces the same output tensors as the forward pass in the original model.

Example usage for random environment data:

```python
from transformers import TrajectoryTransformerModel
import torch
import numpy as np

config = TrajectoryTransformerConfig.from_pretrained("CarlCochet/trajectory-transformer-halfcheetah-medium-v2")
model = TrajectoryTransformerModel.from_pretrained("CarlCochet/trajectory-transformer-halfcheetah-medium-v2", config=config)

batch_size = 1
# sequence made of [ actions, observations, reward ]
seq_length = config.action_dim + config.observation_dim + 1 

trajectories = torch.LongTensor([np.random.permutation(seq_length) for _ in range(batch_size)])
outputs = model(trajectories)

prediction_logits = outputs.logits
```

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?

## TODO

- Pass all relevant tests ; need to disable some irrelevant tests as well
- Make a demo notebook

## Who can review?

Anyone in the community is free to review the PR once the tests have passed.
@LysandreJik @thomwolf @edbeeching 

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Pinging @sgugger for a second review!