[FSDP] Do not `sys.exit(0)` explicitly at end of unit test (#100645)
We are going to see if this closes https://github.com/pytorch/pytorch/issues/100641. The guess is that this might allow NCCL to be destroyed before Python finalizes, avoiding any issues with calling `pybind11::gil_scoped_release` like in [`destroy_nccl_comm`](https://github.com/pytorch/pytorch/blob/8994d9e6109c541a1d581c383e4de9ed68205d91/torch/csrc/cuda/python_nccl.cpp#L46).
Test plan:
```
CUDA_VISIBLE_DEVICES=0,7 numactl -C 2 python test/distributed/fsdp/test_fsdp_unshard_params.py -v -k test_with_grads_core --repeat 200 2>&1 | tee out
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100645
Approved by: https://github.com/rohan-varma, https://github.com/fegin
