Use `ptrdiff_t` rather than `ssize_t` (#71271)
Summary:
`diff_type` kind of naturally should be `ptrdiff_t`, as `ssize_t` is actually defined [here](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_types.h.html) as :
> The type ssize_t shall be capable of storing values at least in the range [-1, {SSIZE_MAX}].
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71271
Reviewed By: atalman
Differential Revision: D33569304
Pulled By: malfet
fbshipit-source-id: 57dafed5fc42a1f91cdbed257e76cec4fdfbbebe
