[LTC] Refactor LTCTensorImpl (#68409)
Summary:
This commit refactors LTCTensorImpl:
1) Adds both const& and && ctors for readability.
2) Removes unused APIs.
3) Keeps coding style to be consistent on the snake case.
4) Adds comments to some unconventional implementations, and consolidates some.
Test Plan:
lazy_tensor_core/test/cpp/build/test_ptltc
