Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70062
This commit upstreams LTCTensorImpl from the lazy_tensor_staging branch.
It inherits from c10::TensorImpl and thus manages the lifetime/storage
of LazyTensor.
Test Plan: ./build/bin/test_lazy --gtest_filter=LazyTensorImplTest.*
Reviewed By: desertfire
Differential Revision: D33171186
Pulled By: alanwaketan
fbshipit-source-id: 6af9f91cc7c7e997f120cb89a7bcd6785c03ace0
