[LTC] Merge and prune some bridge utils
Summary:
This commit merges and prunes some bridge utils:
1. Modifies TryGetLtcTensor to return LazyTensor directly given it's a pointer type anyway.
2. Moves some utils out given they are only useful for a particular file.
3. Adds operator bool() to LazyTensor.
Test Plan:
lazy_tensor_core/test/cpp/build/test_ptltc
