lib: fix Already borrowed

[d4l3k]

This fixes errors from Rust when calling the manager from multiple threads.

[rank1]: Traceback (most recent call last):
[rank1]:   File "/home/mreso/torchft/train_fsdp.py", line 176, in <module>
[rank1]:     main()
[rank1]:   File "/home/mreso/torchft/train_fsdp.py", line 169, in main
[rank1]:     optimizer.step()
[rank1]:   File "/home/mreso/torchft/torchft/optim.py", line 53, in step
[rank1]:     if self.manager.should_commit():
[rank1]:   File "/home/mreso/torchft/torchft/manager.py", line 556, in should_commit
[rank1]:     should_commit = self._client.should_commit(
[rank1]: RuntimeError: Already borrowed

We have the Manager receiver type set as &mut self when it really doesn't need to be. We instead do a lightweight clone of the client when making network calls.

See PyO3/pyo3#2465 for more details

Test plan:

pytest
cargo test

[mreso]

LGTM