Split out Function into its own file (#21539)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21539
ghimport-source-id: f1e4396a0bec6e30d3179f926ec4da68807942f7
Differential Revision: D15741979
Pulled By: suo
fbshipit-source-id: 4cd0ed36bcbf8db0b36a101dda6f58975f806889
