don't materialize constants (#21229)
Summary:
This doesn't affect anything because we run constant pooling, and in the case of Closures and Forks creates unnecessary closures over constants.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21229
Differential Revision: D15587764
Pulled By: eellison
fbshipit-source-id: d5609b0a5697071fab5050eb9e03876ab9ebb27a
