[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: D25338250
Pulled By: malfet
fbshipit-source-id: e840618b113b8bc0d8bb067c2fdf06e3ec9233d4
