Add a context manager for activation sharding.

[luyug]

What does this PR do?

This pr adds a context manager for sharding (potentially long-living) activation saved tensors across model parallel group. It can be used to wrap any non-mp regions where the activations are duplicated across all ranks in a mp group. One specific and typical use case is to wrap a series of gradient checkpoints of transformer layers, creating a layer input sharding setup similar to the one described in the GSPMD paper.

The implementation uses torch.autograd.graph.saved_tensors_hooks and share similar spirits with torch.autograd.graph.save_on_cpu.

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