[FSDP()][17/N] Refactor `_fsdp_root_pre_forward()` (#87930)
This PR moves `_fsdp_root_pre_forward()` to `_runtime_utils.py`.
Note: This PR includes a (temporary) fix for `NO_SHARD` + `CPUOffload(offload_params=True)`, where we set `non_blocking=False` when copying the gradient from device to host. It is only included in this PR since the test was **flaky** (but not consistently failing) on this PR , so I needed to fix to unblock land.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87930
Approved by: https://github.com/mrshenli
