fix document masking for chunked attention
Author: winglian
Pull request: yes

# What does this PR do?

The chunked attention was clobbering the document_ids meaning that the info to distinguish documents within the same chunk is gone. The proper way to do this is to simply generate a chunk mask and combine with the existing causal mask.

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## Who can review?
@ArthurZucker @SunMarc 
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@ArthurZucker Anything I need to do to get this over the line?

Nope sorry was off for two weeks!