[futures] Add c++ ivalue::Future collectAll() helper (#39119)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39119
Add some base c++ unittest coverage for ivalue::Future, and in
the process, add a basic collectAll() primitive, per 38937.
In the process, I realized that List<Future> is effectively
impossible to construct (since the Future's type is not templated,
but rather passed in, the getTypePtr_<T>::call() isn't defined),
so added a workaround in List to make it possible.
ghstack-source-id: 105309650
Test Plan: buck test mode/dev-nosan caffe2/test/cpp/jit/...
Differential Revision: D21756884
fbshipit-source-id: 5d40c8d1c55098de5497655c7b887f4f56508a37
