[TensorPipe] Use targetDevice in tensorpipe_agent. (#56346)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56346
Now that TensorPipe's API has `targetDevice`, use that instead of
manually writing the CUDA device index in `metadata`.
Test Plan: CI
Reviewed By: lw
Differential Revision: D27703235
fbshipit-source-id: c5b620e3b3ce619367412efdbe9fa3778f6b8869
