[future] Avoid some future callback self-captures. (#36502)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36502
We're sometimes deleting futures without completing them (discovered by logging),
and we've recently noticed a slow memory leak.
This change migrates the future lambda cases where there was self-capture.
- In some cases, we use weak_ptr<>, plus .lock()/assert in the lambda callback.
This avoids the reference cycle. We use this primarily in the case where the
value ends up being moved in the callback (something we want to be careful about)
- We also add a convenience api to Future where the completed Future is returned as an arg.
This allows us to avoid self-capture, though it assumes that the markCompleted()
caller is persisting the future for the markCompleted() duration (this has been the case)
ghstack-source-id: 102130672
Test Plan: ctr_mobile_feed, buck test mode/dev-nosan caffe2/test/...
Differential Revision: D20998905
fbshipit-source-id: 7dd52fe4e567a5dea20e8d43862fc2335fd3ce16
