Add callbacks, async checkpoints, and pipeline

[ak495867]

Introduce a lightweight callback system (src/accelerate/callbacks.py) and integrate it into Accelerator (register/trigger helpers and lifecycle hooks). Add asynchronous checkpointing: capture/write split, staging to CPU, and AsyncCheckpointManager (checkpointing.py) to offload disk writes. Add pipeline parallel utilities (src/accelerate/pipeline_parallel.py) with prepare_pipeline and pipeline_forward, and extend ParallelismConfig to support pp_size. Improve optimizer (optimizer.py) to enable fused/foreach defaults where safe and to emit on_optimizer_step callbacks. Misc: wire callbacks into save/load/log/backward flows and minor API/import adjustments.

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Fixes # (issue)

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