Add return to mlx_do_nothing fn

[EnzoDeg40]

Summary:
This pull request proposes a minor modification to the mlx_int_do_nothing function, altering its behavior to return 0. While seemingly inconsequential, this change is made with the intention of ensuring consistency and clarity within the codebase.

Justification:
The mlx_int_do_nothing function, as its name suggests, is designed to perform no operation other than existing as a placeholder or as part of a control flow. Historically, it has lacked a return statement or returned void, indicating its inert nature. However, in the interest of robustness and adherence to best practices, introducing a return statement that explicitly returns 0 serves several important purposes:

1. Clarity and Intent:
   By explicitly returning 0, the function's purpose becomes more evident to developers who may encounter it in the future. This clarity aids in comprehension, particularly in scenarios where the function is invoked within larger codebases or by developers unfamiliar with its context.

2. Defensive Programming:
   While the function itself may not possess any significant side effects or dependencies, it is prudent to establish a clear contract regarding its behavior. Returning 0 communicates to callers that the function executed successfully or, more accurately, that it successfully did nothing. This can prevent unintended consequences or assumptions in downstream code.

3. Compatibility and Standardization:
   Returning 0 aligns with conventions commonly observed in C programming, where functions are expected to return an integer status code indicating success or failure. While the success of mlx_int_do_nothing may seem trivial, conforming to established norms fosters consistency and interoperability within the codebase.

4. Future-Proofing:
   Although the function's current implementation may not necessitate a return value, preemptively incorporating a return statement future-proofs the codebase. Should the function's behavior evolve or dependencies change, having a return value in place facilitates such modifications without disrupting existing interfaces or assumptions.

Conclusion:
In conclusion, while the modification to mlx_int_do_nothing may appear nominal on the surface, it reflects a conscientious effort to enhance the maintainability, clarity, and robustness of the codebase. By introducing a return statement that explicitly returns 0, this pull request aligns with best practices, promotes consistency, and ensures that even in the absence of action, the function communicates its status effectively.