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* fix(transfo_xl): Fixes TransfoXL support when using Trainer.

* fix(tests): Uses losses_1 and losses_2 pattern with TransfoXL test.

* fix(transfo_xl): Adds requested changes to allow for backward compatibility.

fix(transfo_xl): Adds requested changes to allow for backward compatibility.

fix(transfo_xl): Fixes code styling.

* Backward compatibility

* Update src/transformers/models/transfo_xl/modeling_transfo_xl.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Gustavo de Rosa <gth.rosa@uol.com.br>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>