Always use intrusive_ptr for Message (1 out of 2) (#58422)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58422
Similar to Future (which I tackled recently), Message is an ivalue type (a "custom class" one), and the natural way to represent it is inside an intrusive_ptr. However in the RPC code we had a mix of usages, often passing Message by value. This has undesirable consequences, as it could easily trigger a copy by accident, which I believe is why in many places we accepted _rvalue references_ to Message, in order to force the caller to move. In my experience this is non-idiomatic in C++ (normally a function signature specifies how the function consumes its arguments, and it's up to the caller to then decide whether to copy or move).
By moving to intrusive_ptr everywhere I think we eliminate and simplify many of the problems above.
In this PR I do half of the migration, by updating everything except the `toMessageImpl` methods, which will come in the next PR.
ghstack-source-id: 129567053
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D28474878
fbshipit-source-id: 5b76d45e05f6fa58c831e369c5c964d126187a6c
