Add trajectory transformer (#17141)

* Add trajectory transformer


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Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
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Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
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Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
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* Remove tapex

* Remove tapex

* Remove tapex

docs/source/en/model_doc/trajectory_transformer.mdx

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+<!--Copyright 2022 The HuggingFace Team. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+the License. You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+specific language governing permissions and limitations under the License.
+-->
+
+# Trajectory Transformer
+
+## Overview
+
+The Trajectory Transformer model was proposed in [Offline Reinforcement Learning as One Big Sequence Modeling Problem](https://arxiv.org/abs/2106.02039)  by Michael Janner, Qiyang Li, Sergey Levine.
+
+The abstract from the paper is the following:
+
+*Reinforcement learning (RL) is typically concerned with estimating stationary policies or single-step models,
+leveraging the Markov property to factorize problems in time. However, we can also view RL as a generic sequence
+modeling problem, with the goal being to produce a sequence of actions that leads to a sequence of high rewards.
+Viewed in this way, it is tempting to consider whether high-capacity sequence prediction models that work well
+in other domains, such as natural-language processing, can also provide effective solutions to the RL problem.
+To this end, we explore how RL can be tackled with the tools of sequence modeling, using a Transformer architecture
+to model distributions over trajectories and repurposing beam search as a planning algorithm. Framing RL as sequence
+modeling problem simplifies a range of design decisions, allowing us to dispense with many of the components common
+in offline RL algorithms. We demonstrate the flexibility of this approach across long-horizon dynamics prediction,
+imitation learning, goal-conditioned RL, and offline RL. Further, we show that this approach can be combined with
+existing model-free algorithms to yield a state-of-the-art planner in sparse-reward, long-horizon tasks.*
+
+Tips:
+
+This Transformer is used for deep reinforcement learning. To use it, you need to create sequences from
+actions, states and rewards from all previous timesteps. This model will treat all these elements together
+as one big sequence (a trajectory).
+
+This model was contributed by [CarlCochet](https://huggingface.co/CarlCochet). The original code can be found [here](https://github.com/jannerm/trajectory-transformer).
+
+## TrajectoryTransformerConfig
+
+[[autodoc]] TrajectoryTransformerConfig
+
+
+## TrajectoryTransformerModel
+
+[[autodoc]] TrajectoryTransformerModel
+    - forward