perf: add reference_internal policy for const-ref returns

[jashshah999]

Summary

• Methods returning const T& now emit py::return_value_policy::reference_internal in the generated pybind11 bindings
• The generated lambda uses -> const auto& trailing return type to preserve the reference semantics
• This avoids unnecessary copies when Python accesses C++ members by const reference, keeping the returned reference tied to the parent object's lifetime

Motivation

Without this policy, pybind11 defaults to copy for returned references from lambdas (since the lambda return type is deduced as a value). This causes:

1. Unnecessary deep copies of potentially large objects (matrices, vectors)
2. Surprising semantics where modifying the "reference" in Python doesn't affect the C++ object

With reference_internal, pybind11 returns a reference and ties the Python reference's lifetime to the parent object, matching C++ semantics.

Changes

• gtwrap/pybind_wrapper.py: Detect is_ref on the return type and emit -> const auto& + py::return_value_policy::reference_internal for instance methods
• tests/expected/python/class_pybind.cpp: Updated expected output for return_vector2 and return_matrix2 const-ref overloads

Test plan

• All existing tests pass (pytest tests/test_pybind_wrapper.py - 9/9 passing)
• Verify with a downstream project (e.g. GTSAM) that const-ref accessors no longer copy

[dellaert]

@varunagrawal might be too busy - sorry. My comment is: please add a unit test to (a) show me how emitted code differs, and (b) verify that for posterity.

[dellaert]

Thanks for adding the test; it covers the intended const T& case well.

One issue with the implementation: the generator currently checks only return_type.type1.is_ref, while the behavior and comments are specifically about const T&. That means a future method returning a non-const mutable reference, e.g. SomeType& mutableThing(), would also be emitted as -> const auto& with py::return_value_policy::reference_internal. That would accidentally turn a mutable C++ reference into a const reference in the Python binding.

Could you either gate this path on both is_ref and is_const, or add a separate non-const-reference path/test if mutable reference returns are meant to be supported? For this PR as written, the conservative fix is probably to require type1.is_const before emitting -> const auto&.

[dellaert]

Actually, I will add that in a follow up PR. Don;t pull wrap into gtsam yet.