[WIP Fix pipe close warnings

[H-Huang]

I'm seeing the warnings below on program exit. This PR adds the shutdown method to atexit handler and rewrites the shutdown logic to exit cleanly which fixes the warnings.

got unexpected error in future handler: 
Traceback (most recent call last):
  File "/home/howardhuang/local/torchft/torchft/process_group.py", line 1168, in _future_handler
    cmd = future_pipe.recv(timedelta(seconds=10))
  File "/home/howardhuang/local/torchft/torchft/multiprocessing.py", line 21, in recv
    out = self._pipe.recv()
  File "/home/howardhuang/.conda/envs/torchft/lib/python3.10/multiprocessing/connection.py", line 250, in recv
    buf = self._recv_bytes()
  File "/home/howardhuang/.conda/envs/torchft/lib/python3.10/multiprocessing/connection.py", line 414, in _recv_bytes
    buf = self._recv(4)
  File "/home/howardhuang/.conda/envs/torchft/lib/python3.10/multiprocessing/connection.py", line 383, in _recv
    raise EOFError
EOFError
got unexpected error in future handler: 
Traceback (most recent call last):
  File "/home/howardhuang/local/torchft/torchft/process_group.py", line 1168, in _future_handler
    cmd = future_pipe.recv(timedelta(seconds=10))
  File "/home/howardhuang/local/torchft/torchft/multiprocessing.py", line 21, in recv
    out = self._pipe.recv()
  File "/home/howardhuang/.conda/envs/torchft/lib/python3.10/multiprocessing/connection.py", line 250, in recv
    buf = self._recv_bytes()
  File "/home/howardhuang/.conda/envs/torchft/lib/python3.10/multiprocessing/connection.py", line 414, in _recv_bytes
    buf = self._recv(4)
  File "/home/howardhuang/.conda/envs/torchft/lib/python3.10/multiprocessing/connection.py", line 383, in _recv
    raise EOFError
EOFError
[W310 15:51:52.585664080 CudaIPCTypes.cpp:16] Producer process has been terminated before all shared CUDA tensors released. See Note [Sharing CUDA tensors]

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