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Extend BatchSamplerShard (in src/accelerate/data loader.py ) to support variable-length (dynamic) batch…

Extend BatchSamplerShard (in src/accelerate/data loader.py ) to support variable-length (dynamic) batch…: a task in HF ML Tasksmith (Harbor dataset). Current broken behaviour (base branch): constructing BatchSamplerShard with a sampler that has no batch size attribute and even batches=True raises…

The task

**Current broken behaviour (base branch):** constructing `BatchSamplerShard` with a sampler that has no `batch_size` attribute and `even_batches=True` raises `ValueError: "You need to use even_batches=False when the batch sampler has no batch size"`.

Part of FineEnvs/HF_ML_Tasksmith.