[LTC] Supported optional operands in the code-gen and generate native_layer_norm
Summary:
This commit added support for optional operands in the code-gen engine, which basically introduced a named bitmap in each leaf node class to indicate whether the operand exists or not.
So the Clone() method and the lowering code can iterate over operands accordingly.
Test Plan:
lazy_tensor_core/test/cpp/build/test_ptltc --gtest_filter=AtenLtcTsTensorTest.TestLayerNorm
