Need a macro to disable bf16 support?

[leofang]

After #306 CUB supports bf16 in some functions. Unfortunately, this means the bf16 headers (cuda_bf16.h, cuda_bf16.hpp) become a dependency of Thrust/CUB. Let me explain why it is unfortunate.

Normally, this would be fine in the majority of cases, as CUDA headers are also needed at compile time. But in some cases, such as within a CUDA runtime docker (which contains no headers) + using Jitify to compile CUB kernels at runtime, it is a big issue due to the missing headers.

Usually, the community approach is to bundle certain headers and redistribute them, so for example Thrust, CUB, and the fp16 headers (cuda_fp16.h, cuda_fp16.hpp) can be (and are being) repackaged in many projects. The unfortunate point is that the bf16 headers are not redistributable according to the CUDA EULA, and based on an internal conversation it'd remain the case for a long while. Therefore, it would be nice if CUB could offer a macro to disable the bf16 support.

[gevtushenko]

@leofang thank you for reporting the issue! Please, check if the following PR addresses your issue.

[leofang]

Relevant: nvbugs 3641496