[future] Make new IValue future constValue semantics match torch::utils counterpart (#38355)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38355
The torch::utils::Future api which this api was copied from last week
intentionally does not throw. Harmonize the semantics and comment
appropriately.
ghstack-source-id: 104014210
Test Plan: buck test mode/dev-nosan caffe2/test/...
Differential Revision: D21533016
fbshipit-source-id: db26af32656d7b9dacf4fad4e77c944a0087c9b0
