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.
Please also list any relevant details for your test configuration.

• 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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Thanks for contributing 🎉!

[mreso]

LGTM