delete unnecessary file .gitkeep (#18136)
Summary:
delete unnecessary file .gitkeep in /pytorch/tree/master/torch/csrc/nn
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18136
Differential Revision: D14516584
Pulled By: ezyang
fbshipit-source-id: a7555693cb3df1c5e37fcd3ca9bb379a2258f2d1
