feat: support flash attn 2 in Granite MOE
Author: jannisborn
Pull request: yes

# What does this PR do?

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While the GraniteMOE model currently supports flash attention 2 (`supports_flash_attn_2 = True`) it expects the user to alter inputs when FA2 is used. This is counterintuitive and should be fixed. For context, FA2 throws an error if an attention mask is provided. The existing logic (see [here](https://github.com/huggingface/transformers/blob/1580f6465347df4e8d7c12cd4d3dc603b8a689ad/src/transformers/models/granitemoe/modeling_granitemoe.py#L778)) does not ensure that no attention mask is passed if FA2 is used. 
This PR fixes that.



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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: granitemoe

Sorry, I overlooked something on my side. All seems good! Closing this

No worries, glad to hear