Add decisiontransformer to onnx config
# What does this PR do?
Adds an onnx configuration for decision transformer


Fixes # (issue)
https://github.com/huggingface/transformers/issues/18191

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [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).
- [ ] Did you write any new necessary tests?
In progress while PR is being reviewed

## Who can review?
@ChainYo @regisss  Here's a first cut, I will be reading the documentation on testing this while the PR is being reviewed, again appreciate any help you can provide as this is my first time through this process 


The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19189). All of your documentation changes will be reflected on that endpoint.<|||||>@ChainYo @regisss bumping this PR<|||||>> @ChainYo @regisss bumping this PR

It looks good to me!
Thanks for iterating on this, @skanjila.

**EDIT**: Did you do ci tests before pushing? (linting and stuff?)<|||||>@ChainYo @regisss  I reran make style and everything passed locally and also added an entry to DecisionTransformer in the test_onnx_v2.py file, please have a look now and lmk what else is missing<|||||>@skanjila Hmm too many files got reformatted, I think your version of Black is different from what we use. Could you run `pip install transformers["quality"]` and then `make style`?<|||||>@regisss done, and make style worked, please recheck<|||||>@skanjila You also need to run `make fix-copies`, I forgot this one my bad

And there are still many reformatted files, did you remove the files from the previous commit?<|||||>@regisss here is what I did
1) I reset git to an earlier commit before all my changes
2) I added back the entry for the decision_tranformer in test_onnx_v2.py
3) I reran make style after installing transformers["quality"]
4) I ran make fix-copies
5) steps 3/4 were successful

Should be good to go, please have a look , I look forward to finishing this work.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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