Fix hsdp_device_mesh=None when enable HSDP and HYBRID_SHARD

[haozhx23]

What does this PR do?

Fixes # A variable hsdp_device_mesh is assigned None before the same-name function hsdp_device_mesh() is called, when hsdp and HYBRID_SHARD are both set. Change the variable hsdp_device_mesh to hsdp_device_mesh_plan to avoid conflict.

Feature/Issue validation/testing

Please describe the tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
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• Test 1
  TypeError("'NoneType' object is not callable") fixed.

• An error from torch/distributed/fsdp/_init_utils.py L107 will be triggered.

            "Cannot pass both process_group and device_mesh at the "
            "same time. Please just pass only one of them."

As neither process_group or device_mesh is None. With a temporary modification, the training will proceed (with both HSDP and HYBRID_SHARD enabled)

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