Rename shared_ptr<SymIntNodeImpl> to SymIntNode (#82355)
Makes code a lot more compact! It also makes it possible to swap out
the shared ptr implementation, which I am about to do next.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82355
Approved by: https://github.com/Krovatkin
