[TensorPipe] Don't use separate heap allocation for metrics (#39183)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39183
I didn't see any reason behind it, and it seems to work even after removing the unique_ptrs. (Well, it compiles...)
ghstack-source-id: 104837027
Test Plan: None...
Differential Revision: D21767863
fbshipit-source-id: daebfae69d5b63f1d10345abd625b7e0ddce7e6d
