Get rid of {Cpu,Cuda}{Channel,Context} in tensorpipe_agent. (#54432)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54432
Following the merge of channel hierarchies, here comes the promised
clean up.
Test Plan: CI
Reviewed By: lw
Differential Revision: D27232442
fbshipit-source-id: 540dc6bc18a9a415b676e06e75530d729daf2d5b
