Adds `TRANSFORMERS_TEST_BACKEND` (#25655)
* Adds `TRANSFORMERS_TEST_BACKEND`
Allows specifying arbitrary additional import following first `import torch`.
This is useful for some custom backends, that will require additional imports to trigger backend registration with upstream torch.
See https://github.com/pytorch/benchmark/pull/1805 for a similar change in `torchbench`.
* Update src/transformers/testing_utils.py
Co-authored-by: Yih-Dar <2521628+ydshieh@users.noreply.github.com>
* Adds real backend example to documentation
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Co-authored-by: Yih-Dar <2521628+ydshieh@users.noreply.github.com>
