Support save/load ckpt for XLA FSDP (#32311)
* Support save/load ckpt for XLA FSDP
* Fix bug for save
* Fix style
* reserve sharded ckpt and better file naming
* minor fix
Co-authored-by: Zach Mueller <muellerzr@gmail.com>
* add is_fsdp_xla_v1_enabled
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Co-authored-by: Zach Mueller <muellerzr@gmail.com>