feat: Restrict __slots__=() for subclasses of tvm_ffi.Object by default

[junrushao]

This PR makes __slots__=() for all subclasses of tvm_ffi.Object if not specified. Here's all the changes:

• Rename the existing Cython class Object to CObject
• Create a new Object whose metaclass is _ObjectSlotsMeta
• Encourage inheriting from the new tvm_ffi.Object

[gemini-code-assist]

Summary of Changes

Hello @junrushao, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant refactoring to the tvm_ffi.Object class structure, primarily to enforce the use of __slots__=() across its inheritance chain. This change is designed to optimize memory consumption and enhance the performance of attribute access for FFI objects by preventing the creation of instance dictionaries. The refactoring involves renaming the internal Cython base class, establishing a new Python-level Object class with a metaclass to manage __slots__ policy, and updating all dependent Cython modules to align with the new hierarchy and slot declarations.

Highlights

• Refactored Object Hierarchy: The core tvm_ffi.Object class hierarchy has been refactored. The original Cython Object class is renamed to CObject, and a new Python Object class is introduced which inherits from CObject.
• Enforced __slots__=(): A new metaclass, _ObjectSlotsMeta, is introduced for the tvm_ffi.Object class. This metaclass automatically enforces __slots__=() for all subclasses of tvm_ffi.Object if not explicitly defined, and prevents the inclusion of __dict__ in __slots__.
• Optimized Attribute Access for Module: The Module class now explicitly uses __slots__ and implements a caching mechanism for function attributes in its __getattr__ method, improving performance for repeated attribute lookups.
• Updated Cython Classes: Numerous Cython classes that inherit from Object (now CObject) have been updated to explicitly declare __slots__ = () and their type hints and casts have been adjusted to use CObject where appropriate.

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[gemini-code-assist]

Code Review

This pull request introduces a significant and valuable refactoring to enforce the use of __slots__ for all subclasses of tvm_ffi.Object. This is achieved by renaming the Cython Object to CObject and introducing a new Python Object with a metaclass that automatically adds __slots__ = (). This change will lead to considerable memory savings by preventing the creation of __dict__ for FFI objects.

The implementation is well-executed, with consistent updates across the codebase. The use of a metaclass with __instancecheck__ and __subclasscheck__ is a clever solution to bridge Python and Cython inheritance. The tests have also been updated to validate the new behavior.

I've identified a minor issue where some Cython classes have __slots__ = () added but retain a cdef dict __dict__ declaration. This seems contradictory to the goal of the PR and should be cleaned up to realize the full memory benefits.