Replace lazy_tensors::Span with c10::ArrayRef
Summary: This is a preparation step for landing on master. Because
c10::ArrayRef does not take const qualifier, we need to drop them in
this rewrite, which should be safe since ArrayRef only provides const iterator.
TODO: There is still use of Span in class Literal, where the Span needs to be
mutable indeed. Let's revisit this when working on the Literal class.
