lazy import torch._softmax_backward_data for better compatibility
Author: daquexian
Pull request: yes

# What does this PR do?

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Dear huggingface team,

Thanks for the great library! I'm from [OneFlow](https://github.com/Oneflow-Inc/oneflow), a deep learning framework with PyTorch-compatible APIs and better performance. We want OneFlow users to run 3rd libraries by simply replacing all `torch` with `oneflow`. For `transformers` library, a blocker is the import of internal API `torch._softmax_backward_data` (OneFlow doesn't have the same internal APIs with PyTorch). This PR moves the import from the global scope into the function `softmax_backward_data`, so in most cases it will not be triggered.


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@fxmarty @sgugger

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@sgugger thanks for your review! Tests all pass now