Remove LazyStreamContext (1 out of 2) (#59298)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59298
After recent changes, LazyStreamContext had in fact always become eager, and was in fact equivalent to a vector of streams. So it makes more sense now to remove this abstraction and use a more self-descriptive type.
This PR migrates the RequestCallback internals. The next PR migrates the TensorPipe agent.
ghstack-source-id: 130583774
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D28789175
fbshipit-source-id: fa581a50f9a6a1e42c2ad8c808a9b099bea7433e
