Call `abort` when tearing down `ProcessGroupNCCL`

[d4l3k]

In PyTorch 2.7 we'll have new abort and shutdown methods exposed. We should call these when using torchft's ProcessGroupNCCL and reconfiguring to destroy the old communicators.

pytorch/pytorch#148798

https://github.com/pytorch/torchft/blob/main/torchft/process_group.py#L354-L356