[FSDP] Change _create_chunk_dtensor in fsdp/_shard_utils.py to use public API from DTensor (#110831)
This PR:
1) updates _create_chunk_dtensor() in _shard_utils.py to use public APIs from DTensor. This will avoid the global_size calculation error from using DTensor.from_local() for uneven-sharded parameters, as described in https://github.com/pytorch/pytorch/issues/110762
2) updates test/distributed/fsdp/test_fsdp_dtensor_state_dict.py to include unit test for a model with uneven sharding.
cc. @wanchaol, @fegin
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110831
Approved by: https://github.com/wanchaol, https://github.com/fegin
