add FlashAttentionKwargs and seq_idx to flat collator

[garrett361]

What does this PR do?

Adds additional, optional return values in DataCollatorWithFlattening as needed for padding-free training with particular models.

Relates to #35861 and #35941.

Fixes # (issue)

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[vasqu]

I'd like to take a look as well when you think you're ready, so gladly ping then :)

[vasqu]

cc @ArthurZucker