[Easy][FSDP] Remove variable shadowing (#82386)
I unintentionally had the sub-tensors returned by `split()` bound to `tensor`, which shadows the original full `tensor`. This is a bad practice and is error-prone.
Test Plan: CI
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82386
Approved by: https://github.com/rohan-varma
