Fix training speed regression introduced by "optimize VRAM for calculating pos_bias in LayoutLM v2, v3 (#26139)"
Author: kkoehncke
Pull request: yes

# What does this PR do?

This PR aims to fix a training speed regression introduced by a7e0ed829c398a67a641a401e23dae13e2f8b217, in the LayoutLMv2 / v3 family of models. By adding `torch.no_grad()` contexts to the indexing operations (which remain constant throughout training), the training speed is reduced back to how it was before this commit.

Please see linked issue for more details on how to re-produce issue.

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Fixes #30974 (issue), regression in the training speed of LayoutLMv2 / v3

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Comments:
@ArthurZucker as per our discussion on https://github.com/huggingface/transformers/issues/30974, instead of reverting, wrapping the indexing operations in `torch.no_grad()` allows to continue to benefit from the initial commit's VRAM savings.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_30988). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

thanks a lot! great work 🤗