Port FractionalMaxPool2d from TH to ATen (#15531)
Summary:
Tested:
pytest test/test_nn.py -k Fractional
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15531
Differential Revision: D13612833
Pulled By: chandlerzuo
fbshipit-source-id: b919d698d068b97ba7a4f8021367e7f6c8aae39c
