[FSDP2] Relaxed check for parent mesh (#121360)
Mixing 1D and 2D `DTensor`s in the same sharded state dict should be okay, so we can remove the check that a parameter for FSDP to shard must be a `DTensor` if passing a child mesh to FSDP.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121360
Approved by: https://github.com/yifuwang, https://github.com/Skylion007
ghstack dependencies: #120351, #121328
