fix pytorch mobile build (#29414)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29414
add a missing file and fix a std::to_string call.
Test Plan: buck build //xplat/caffe2:torchAndroid#android-armv7,shared
Reviewed By: ljk53
Differential Revision: D18351498
fbshipit-source-id: 41225bff974058eef485a9991d0cc16c67a4074a
