[LTC] Add support for code-gen operators that have tuple output
Summary:
This patch added support for code-gen operators that have tuple output.
1) It changed Node::at_dtypes_ & Node::at_shapes_ to std::vector so now they are generalized to describe tuple output.
2) It added utilities to convert tuple to vector and vice versa.
3) It teached the code-gen engine to tell such an operator and utilize the above utilities.
4) It then generated topk.
Test Plan:
lazy_tensor_core/test/cpp/build/test_ptltc --gtest_filter=AtenLtcTsTensorTest.TestTopK
