remove default constructor in futureInfo (#30197)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30197
This default constructor was added because std::map's operator[]
requires a default constructor. However, instead of using operator[], we can
use emplace and remove the constructor, to ensure that the FutureInfo struct
doesnt get constructed with garbage values.
ghstack-source-id: 94802453
Test Plan: Unit tests pass.
Differential Revision: D18627675
fbshipit-source-id: c4cb000e60081478c0fd7308e17103ebbc4dc554
