Let RpcAgent::send() return JitFuture (#49906)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49906
This commit modifies RPC Message to inherit from `torch::CustomClassHolder`,
and wraps a Message in an IValue in `RpcAgent::send()`.
Test Plan: Imported from OSS
Reviewed By: lw
Differential Revision: D25719518
Pulled By: mrshenli
fbshipit-source-id: 694e40021e49e396da1620a2f81226522341550b
