fix: skip dropout in eval for flash_attn in various models
Author: fdschmidt93
Pull request: yes

# What does this PR do?

For `M2M100FlashAttention2` was not skipped when `model.eval()` was called, which of course quite significantly affects model outputs. This PR fixes the issue by setting dropout to `0.0` when `M2M100FlashAttention2.training == False`.

E: It turns out this affected all models that copied their FA2 implementation from Bart.

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Comments:
I have no idea why the FA2 implementation doesn't use a `# Copied from` statement from bart here tbh. Nevertheless, it also reveals this dropout issue in any bart related model / attn implementation. For reference: https://github.com/huggingface/transformers/blob/570c89625b329a2d33ecbfb972a9aae015535471/src/transformers/models/bart/modeling_bart.py#L403

Might be worth to create a superceding PR for bart and add the missing "copied from" for m2m_100 over here.

cc @amyeroberts @ArthurZucker 

Thx for the quick fix! :)

You can run `make fix-copies` and `make style` which handle the "copied from" mechanism and linting respectively. 

And just like hf bot says, you can push an empty commit at the end with `[run-slow] bart, m2m_100` (I doubt we need to test the other copied from models but correct me if I'm wrong 👀). A maintainer will handle those slow runs when they have time.

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Thanks! Hopefully everything addressed now :)

Thanks @vasqu as well for landing a hand! 

🤗 


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