[FSDP][2/N] Remove `_fsdp_wrapped_module.flat_param` (#86122)
This removes **direct** usages of `_fsdp_wrapped_module.flat_param` with `_handles[0].flat_param`. The preferred way to access the `flat_param` will be through the handle. We may converge to only storing `self._handles` and no longer `self.params` in the future. Right now, `self.params` is always exactly `[handle.flat_param for handle in self._handles]`.
cc @mrshenli @pritamdamania87 @zhaojuanmao @satgera @rohan-varma @gqchen @aazzolini @osalpekar @jiayisuse @H-Huang @kwen2501
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86122
Approved by: https://github.com/zhaojuanmao
