[FSDP] Remove `self.numel_padded_per_param` (unused) (#80002)
The list `self.numel_padded_per_param` is constructed but never used. This PR removes it.
Differential Revision: [D37726057](https://our.internmc.facebook.com/intern/diff/D37726057)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80002
Approved by: https://github.com/rohan-varma
