masked_fill is the point-wise operator so porting if from the TH code to Aten (and TensorIterator) expected to be easy. Such migration will help to clean up the code, simplify dispatch as well as provide immediate 2-3x operator performance gain.

Porting guide: https://github.com/pytorch/pytorch/wiki/TH-to-ATen-porting-guide

Example PR with porting of the adaptive_avg_pool2d: #14714

How to use TensorIterator: https://github.com/pytorch/pytorch/wiki/How-to-use-TensorIterator