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…