[reland] Always use intrusive_ptr for Message (2 out of 2) (#59206)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59206
Reland of https://github.com/pytorch/pytorch/pull/58423
This is part 2 of the previous PR. Here we address the remaining occurrences of "raw" Message, namely the ones within toMessageImpl. And since they're the last ones, we make the constructor of Message private, to prevent new usages from emerging.
ghstack-source-id: 130202848
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D28623892
fbshipit-source-id: f815cf6b93e488c118e5d2298473e6e9d9f4c132
