fix -Wsign-compare warnings for some files inside c2 (#18123)

fix -Wsign-compare warnings for some files inside c2 (#18123)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18123

the motivation of this fix is to resolve things like:
for(auto i = 0; i < N; i++) where N is bigger than int32

These instances of comparison were found by enabling -Wsign-compare

There are way too many things to fix, so issuing this as a series of fixes

The plan is to fix all these issues and then enable this flag into Caffe2 to catch future instances

Reviewed By: ZolotukhinM

Differential Revision: D14497094

fbshipit-source-id: bca3927a2188bd33a508fa503ba221c220cdaefe