[shardformer] support module saving and loading

[FrankLeeeee]

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Fixed #4061

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This PR added support for state_dict and load_state_dict for shardformer layers. The work can be summarized as

1. refactored the dtensor API to only use torch.Tensor instead of creating a subclass.
2. implemented and created utility functions for distributed tensors
3. implemented state_dict and load_state_dict for parallel module
4. added tests for weight loading and saving for shardformer layers.

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[FrankLeeeee]

Some test screenshot: