Add WeakIValue, use in tracer. (#21515)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21515
ghimport-source-id: 7898a68791db2b5050164ab01d6ca6991e05746d
Reviewed By: suo
Differential Revision: D15719981
Pulled By: zdevito
fbshipit-source-id: 42cf26cf6541bcdf95f1343da3b9228fe2c229da
