Forward declare half types in cuda::ptx (#2981)

Author: ahendriksen

libcudacxx/include/cuda/__ptx/instructions/cp_reduce_async_bulk.h

@@ -28,16 +28,12 @@
 
 #include <nv/target> // __CUDA_MINIMUM_ARCH__ and friends
 
-#if defined(_LIBCUDACXX_HAS_NVFP16)
-#  include <cuda_fp16.h>
-#endif // _LIBCUDACXX_HAS_NVFP16
-
-#if defined(_LIBCUDACXX_HAS_NVBF16)
-_CCCL_DIAG_PUSH
-_CCCL_DIAG_SUPPRESS_CLANG("-Wunused-function")
-#  include <cuda_bf16.h>
-_CCCL_DIAG_POP
-#endif // _LIBCUDACXX_HAS_NVBF16
+// Forward-declare __half and __nv_bfloat16. The cuda_fp16.h and cuda_bf16.h are
+// expensive to include. The APIs use only pointers, so we do not have to define
+// the types. If the user wants to use these types, it is their responsibility
+// to include the headers.
+struct __half;
+struct __nv_bfloat16;
 
 _LIBCUDACXX_BEGIN_NAMESPACE_CUDA_PTX