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  TITLE
  [Tentative] Adds support for exporting TransformerXL-based models to ONNX
  
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  # What does this PR do?
  
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  Hello everyone! I hope everything is going well with you.
  
  This PR adds a naïve support when trying to export a TransformerXL model with ONNX. Nevertheless, this is a tentative PR as PyTorch still does not support the newest ONNX opset versions (13+), which includes `triu` and `tril` operators (required by TransformerXL).
  
  As soon as PyTorch updates its API, any reflected change will also be updated in this PR. Meanwhile, this tentative PR serves as a guide to anyone that needs to export their model.
  
  **Note:** there is a trick to leverage `triu` and `tril` operators by hand-implementing them with a compatible format to ONNX, instead of using the ones PyTorch's provides.
  
  Best regards,
  Gustavo.
  
  ## 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/master/CONTRIBUTING.md#start-contributing-pull-requests),
        Pull Request section?
  - [ ] 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.
  - [X] Did you make sure to update the documentation with your changes? Here are the
        [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
        [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
  - [X] Did you write any new necessary tests?
  
  
  ## Who can review?
  
  Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
  members/contributors who may be interested in your PR.
  
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  Models:
  
  - albert, bert, xlm: @LysandreJik
  - blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
  - longformer, reformer, transfoxl, xlnet: @patrickvonplaten
  - fsmt: @stas00
  - funnel: @sgugger
  - gpt2: @patrickvonplaten, @LysandreJik
  - rag: @patrickvonplaten, @lhoestq
  - tensorflow: @LysandreJik
  
  Library:
  
  - benchmarks: @patrickvonplaten
  - deepspeed: @stas00
  - ray/raytune: @richardliaw, @amogkam
  - text generation: @patrickvonplaten
  - tokenizers: @n1t0, @LysandreJik
  - trainer: @sgugger
  - pipelines: @LysandreJik
  
  Documentation: @sgugger
  
  HF projects:
  
  - datasets: [different repo](https://github.com/huggingface/datasets)
  - rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
  
  Examples:
  
  - maintained examples (not research project or legacy): @sgugger, @patil-suraj
  - research_projects/bert-loses-patience: @JetRunner
  - research_projects/distillation: @VictorSanh
  
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