[BugFix][TIRx] Return rewritten nodes from Python mutator defaults

[anerli]

Problem and root cause

Python subclasses of PyStmtExprMutator that delegate to a default visitor receive None instead of the rewritten expression or statement. The C++ default visitors invoke their dispatch tables but discard the returned nodes.

Change

• src/tirx/ir/py_functor.cc: return PrimExpr and Stmt from the default visitors and propagate the dispatch-table results.
• tests/python/tirx-base/test_tir_python_mutator_defaults.py: cover delegated expression and statement visitors, checking rewritten children, attribute values, and statement bodies.

Tests and validation

Both new regression tests fail on unmodified upstream because the result is None. With this isolated change, the following command passes 7 tests, 0 skipped:

PYTHONPATH=$PWD/python python -m pytest -q \
  tests/python/tirx-base/test_tir_python_mutator_defaults.py \
  tests/python/tirx-base/test_tir_stmt_functor_ir_transform.py \
  tests/python/tirx-base/test_tir_stmt_functor_substitute.py
pre-commit run --files src/tirx/ir/py_functor.cc \
  tests/python/tirx-base/test_tir_python_mutator_defaults.py

Validation environment: Apple M4 Max, macOS/Darwin 24.5.0 arm64, Python 3.12.11. Each branch was rebuilt separately from upstream 907a88c87 with:

cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release \
  -DUSE_METAL=ON -DUSE_LLVM=OFF -DUSE_CUDA=OFF -DUSE_Z3=OFF
cmake --build build --target tvm tvm_compiler tvm_runtime -j 8

git diff --check passes. The applicable formatting and other pre-commit hooks pass. The repository-wide ASF-header hook reports 11 existing violations, reproduced on unmodified upstream; none is in the changed files.

Related

Independent of BF16 Let/Bind legalization in #70. No duplicate open PR was found.