Unblock Llama2 ONNX export w/ sdpa by falling back to manual impl
Author: BowenBao
Pull request: yes

# What does this PR do?

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Unblocks Llama2 ONNX export with sdpa by falling back to manual implementation.

> ValueError: Attention using SDPA can not be traced with torch.jit.trace when no attention_mask is provided. To solve this issue, please either load your model with the argument attn_implementation="eager" or pass an attention_mask input when tracing the model.

Fixes #28610


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@fxmarty 

Comments:
Hi @ArthurZucker, I have validated the issue is fixed under your PR, thanks! Do you have an ETA when it will get merged? Our workstreams have been blocked by this issue for a while, we need to resolve this export issue asap.

This week 😉 Waiting for @gante's green light and will merge #27931 (it was not clear)

I don't understand why this change is necessary. The error that is normally raised
```
ValueError: Attention using SDPA can not be traced with torch.jit.trace when no attention_mask is provided. To solve this issue, please either load your model with the argument attn_implementation="eager" or pass an attention_mask input when tracing the model.
```
explicitly gives a solution.

@ArthurZucker @BowenBao I believe we can close this issue now that #27931 was merged