In ProcessGroupAgent, use non-iostream torch::load()/save(). (#28063)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28063
Avoid using the iostream versions of torch::load()/torch::save(), which
incur at least one additional full data copy.
ghstack-source-id: 92059608
Test Plan: buck test mode/dev-nosan caffe2/test/...
Differential Revision: D17945206
fbshipit-source-id: ba24376c13762a28e569530e3b1a939ac6f72f43
