Adding maskrcnn model from torchvision (#547)
Summary:
Add maskrcnn model from torchvision.
This is the first step towards adding dynamic shapes support for maskrcnn.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/547
Reviewed By: fmassa
Differential Revision: D32437904
Pulled By: xuzhao9
fbshipit-source-id: 3177b8a70fb8922e36aa2894db8e8d21dabe543b
