Move UndefinedTensorImpl to c10 (meh) (#14817)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14817
unfortunately, we still need this.
Reviewed By: ezyang
Differential Revision: D13348041
fbshipit-source-id: e8dcc89f5c71bd1ea2c9813990dac6e58e63b1fd
