CUDA: topk-moe: add optional parameter for gpt-oss

[am17an]

While looking at this kernel I realized that it is relatively easy to add it for gpt-oss, which does the softmax after the top-k.

Performance on a 4090:

Model	Test	t/s master	t/s cuda_gpt_oss_opt	Speedup
gpt-oss 20B MXFP4 MoE	tg32	170.99	177.68	1.04
gpt-oss 20B MXFP4 MoE	tg64	168.75	175.36	1.04
gpt-oss 20B MXFP4 MoE	tg128	167.01	173.33	1.04