[BE] Fix signed-unsigned warnings (#48848)
Summary:
Switch to range loops when possible
Replace `ptrdiff_t`(signed type) with `size_t`(unsigned type)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48848
Reviewed By: walterddr
Differential Revision: D25376591
Pulled By: malfet
fbshipit-source-id: 9835f83b7a17b6acc20731cc89c1c11c2aa01a78
