Feat: Support dynamic batch size in BatchSamplerShard with even_batches

[yuxinyuan]

What does this PR do?

BatchSamplerShard previously forces uneven batches (even_batches=False) for dynamic batch sizes (e.g. batch_size is None). This PR extends it to work correctly with samplers that yield variable-length batches (i.e., batch_size=None), while preserving all existing behavior.

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[yuxinyuan]

@SunMarc Can you take a look

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the contributing guidelines are likely to be ignored.

[yuxinyuan]

Anyone has time to look at this ?

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the contributing guidelines are likely to be ignored.

[SunMarc]

Thanks ! I'm just curious when you happened to need this

[yuxinyuan]

Usually batch_size is None means the batch sampler produces dynamic sized batch of data.

The BatchSamplerShard already supports batch_size is None case. But it requires even_batches to be False (no loop back at the end of iteration). This PR just removes this constraint (even_batches can be True after merging this PR).