Adding gradient to Boolean Mask operator (#21423)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21423
- add gradient for boolean mask
- add test for gradient checking
Reviewed By: BIT-silence
Differential Revision: D15640036
fbshipit-source-id: 79f40c6901e805bf1b8e9b01b57903e30b00f654
