[Code Standard] Establish a Coding Standard for Return Values vs. Output Parameters in CUDA Kernels

[yewentao256]

I prefer the return, it's more functional, looks more clear, and I think it's a better general c++/CUDA practice, and the compiler is nominally better at optimizing it (in general - here I think everything will be inlined anyway). So I always default to returning.

Originally posted by @ProExpertProg in #19233 (comment)

The vLLM codebase currently employs two different patterns for handling function outputs within our CUDA C++ code:

1. Return Values: Functions return the result directly (e.g., OutType func(InType)).
2. Output Parameters: Functions take a non-const pointer or reference to fill with the result (e.g., void func(InType, OutType*)).

While both are valid, the lack of a formal standard can lead to inconsistencies. Adopting a clear coding standard would improve readability, maintainability, and make it easier for new contributors to understand and write idiomatic code.

I would like to open this topic for discussion with the community. What are your thoughts on this? If we can reach a consensus, we can create a small guide in our contribution documentation and identify existing code that could be refactored in follow-up issues.

[ProExpertProg]

Google Style Guide also recommends returning by value:

Prefer to return by value or, failing that, return by reference. Avoid returning a raw pointer unless it can be null.

CppCoreGuidelines as well:

F.20: For “out” output values, prefer return values to output parameters

More forum questions:

• https://stackoverflow.com/questions/33994995/which-is-more-efficient-return-a-value-vs-pass-by-reference
• https://softwareengineering.stackexchange.com/questions/283925/should-i-set-values-using-the-return-or-by-giving-a-reference
• https://www.reddit.com/r/cpp_questions/comments/1degszr/return_by_value_vs_return_by_reference/
• https://stackoverflow.com/questions/47790909/which-is-better-returning-tuple-or-passing-arguments-to-function-as-references