Make futures vector a local function var (#36677)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36677
Move the `futures` vector to be a local function var like `errorFutures`. Holding the lock to clear the vector is now unnecessary.
ghstack-source-id: 102265569
Differential Revision: D20884589
fbshipit-source-id: c9a13258bee737d86f9b0d11cdd28263bb923697
