[LTC] Adopt upstreamed version of LazyTensor and etc (#70860)
Summary:
This commit adopts upstreamed version of LazyTensor, LTCTensorImpl, LazyGraphExecutor
and bridges. In addition, it removes backend_registrar.cpp as it is duplicated with
backend_interface.cpp.
Test Plan:
./lazy_tensor_core/test/cpp/build/test_ptltc
