fix(py_class): propagate parent total_size through empty intermediate classes

[junrushao]

Summary

• Bug: In Python-defined type hierarchies, when an intermediate parent class
  had no own fields (e.g., GrandParent(x, y) -> Parent(pass) -> Child(a, b)),
  TypeInfo.total_size fell back to sizeof(TVMFFIObject) (~24 bytes) for the
  empty parent, ignoring inherited fields. This caused child field offsets to
  overlap with grandparent field offsets, leading to silent data corruption on
  read/write.
• Fix: Replace the sizeof(TVMFFIObject) floor with
  self.parent_type_info.total_size in TypeInfo.total_size, ensuring inherited
  field space propagates correctly through arbitrarily deep empty parents. Added a
  guard that parent_type_info must be present and an assertion that the base size
  is at least sizeof(TVMFFIObject).
• Tests: Added 6 regression tests in TestFieldInheritance covering offset
  correctness, total_size propagation, non-overlapping child offsets, two-level
  empty intermediate parents, mutation aliasing, and the @py_class decorator path.

Architecture

• Changed file: python/tvm_ffi/cython/type_info.pxi — TypeInfo.total_size
  property body (6 lines replaced with 6 lines; decorator unchanged).
• Root cause: The old code initialized end = sizeof(TVMFFIObject) and only
  iterated over self.fields (the type's own fields). An empty intermediate class
  has no own fields, so it returned a total_size equal to just the object header,
  erasing all inherited field space.
• Fix mechanism: end = self.parent_type_info.total_size inherits the parent's
  full layout (including transitively inherited fields), then max(end, f.offset + f.size)
  extends it for any own fields.

Behavioral Changes

• Before: Child.a at offset 24 overlapping GrandParent.x at offset 24 when
  Parent had no own fields. Reading obj.x returned obj.a's value.
• After: Child.a starts at or after Parent.total_size (which equals
  GrandParent.total_size), so no field overlap occurs.

Test Plan

• uv run pytest -vvs tests/python/test_dataclass_py_class.py::TestFieldInheritance -- all tests pass (8 existing + 6 new)
• All pre-commit hooks pass

[gemini-code-assist]

Code Review

This pull request ensures that total_size is correctly propagated through empty intermediate parent classes in the TVM FFI by updating the property logic in type_info.pxi and adding regression tests. The feedback suggests optimizing the Cython implementation by explicitly typing the parent_size variable as int64_t to reduce Python overhead.