CUDA: add a fused top-K MoE kernel (#16130)

* CUDA: add a fused top-K MoE kernel

This kernel does the following:
1. softmax over the logits per token [n_experts, n_tokens]
2. argmax reduce over the top-k (n_experts_used) logits
3. write weights + ids to global memory

It is intended as fusion of softmax->top-k->get_rows pipeline for MoE models

* Refactor into ggml_cuda_should_use_topk_moe

* Review: Use better coalescing pattern, use WARP_SIZE, store logits into registers before

* Review: format + micro-optimizations

* Fix bug: fix tie breakers

* Add optional norm + clean-up code

* Use smem for final write

* Add bounds check

* Use better memory pattern for writeback

ggml/src/ggml-cuda/topk-moe.cuh

@@ -0,0 +1,14 @@
+#include "common.cuh"
+#include "ggml.h"
+
+#include <initializer_list>
+
+void ggml_cuda_op_topk_moe(ggml_backend_cuda_context & ctx,
+                           const ggml_tensor *         logits,
+                           ggml_tensor *               weights,
+                           ggml_tensor *               top_k,
+                           const bool                  with_norm);
+
+bool ggml_cuda_should_use_topk_moe(const ggml_tensor * softmax, const ggml_tensor * weights);
+
+std::initializer_list<enum ggml_op> ggml_cuda_topk_moe_ops(bool with_norm);