Move fill and fill_diagonal to Fill.cpp, Fill.h, and FillKernel.{cpp,cu}
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/22758
Test Plan: Imported from OSS
Differential Revision: D16257781
Pulled By: ezyang
fbshipit-source-id: 9e5ed06e95ef65b036eb388488faad981f1e8012
