skip PathManager stuff for NFS save

Created by: zdevito

It seems to cause problems:

line 254, in <lambda>
    self._notify_manager(lambda: self._io.write(b))
OSError: [Errno 14] Bad address

But it isn't even needed for saving while also doing training. We can just use concurrent.futures.ThreadPoolExecutor to do the copies from another thread.

Note: how do I verify this doesn't upset the training process timings?