Use intrusive_ptr for LazyTensor (#73445)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73445
Refactors the whole codebase to use LazyTensorPtr (defined as c10::intrusive_ptr) to enable XLA to use a derived class XlaLazyTensor and override functionality.
this PR is just the first step, and we will need to add a factory class that XLA can override in their backend to actually hook up their derived tensor class.
Parallel PR on lazy_tensor_staging: #73429
Test Plan: tested via lazy_tensor_staging test_ptltc and torchbench and CI
Reviewed By: ezyang
Differential Revision: D34481918
fbshipit-source-id: 01176b127df6b79039aa1bc57bc6da5505161f87
(cherry picked from commit 52b9ae4e22d2703d44c6436311d79d40bd62c6aa)
