Use new attention API for LayoutLMv3 (SDPA, Flash Attn v2 support)
Author: stancld
Pull request: yes
Labels: name: SDPA

name: Multimodal

# What does this PR do?

Closes #35467.

## Performance benchmark

Speed & memory req consumption on a token classification ntraining of LayoutLMv3-like model with multilingual support, various auxiliary tasks, masked language modelling.

GPU: 1x A100 80 GB
Batch size: 16, Accumulated gradient batches: 8

| Impl. | Speed | Peak memory |
| --- | --- | --- |
| Eager | ~2.0 it/s | 66.7 Gi |
| SDPA | ~3.0 it/s | 47.2 Gi |

Overall, ~50% speed-up and memory reqs reduction is observed.

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- [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.

cc: @ArthurZucker

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Comments:
Just pinging @NielsRogge or @ArthurZucker .)

@qubvel Thanks for the notes :] Will run some speed benchmarks with various seq lens & batch sizes tonight and add to the docs :]

@ArthurZucker It required rebase, dunno why it looked like those unrelated files were touched.

Will check the new API.

@ArthurZucker Flash Attn impl is still broken here. Will have a look as time allows .]

On nit solve conflicts and good to go!