[FSDP] Use `_get_module_state(module)` in the hook instead of `partial` when registering state dict hooks

[awgu]

#90777 (comment)

cc @mrshenli @pritamdamania87 @zhaojuanmao @satgera @rohan-varma @gqchen @aazzolini @osalpekar @jiayisuse @H-Huang @kwen2501