restore default constructor of OutputArchive (#24955)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24955
Some third-party code relies on this default constructor. It's not
invalid to construct an OuputArchive with an indepednent CU, so
restoring it.
Test Plan: Imported from OSS
Differential Revision: D16935254
Pulled By: suo
fbshipit-source-id: 40b6494e36d10c5009b3031648bee96b2e38b49a
