CUDA: dedup MoE gate/up activation quantization (#25441)

* CUDA: dedup MoE gate/up activation quantization (fp4)

For MoE gate/up projections the src1 activation is broadcast across the
routed experts (ne11 == 1), so ids_src1 maps every one of a token's
n_expert_used slots to the same physical row. The MMQ path therefore
re-quantized each token's activation n_expert_used times.

For fp4 (NVFP4/MXFP4) src0, quantize each unique token row once instead of
once per expert. For NVFP4 a single quantize+scatter kernel
(quantize_scatter_mmq_nvfp4) quantizes each token once and writes the
resulting block_fp4_mmq straight to all n_expert_used slots, using an
inverse token->compact-row map (build_tok2c). MXFP4, and
GGML_CUDA_MOE_QUANT_GATHER=1, use a two-kernel variant: quantize unique
rows then gather into the expert-sorted layout (gather_mmq_fp4_blocks).
Both are bit-identical to the previous gather-then-quantize path (identical
source data, deterministic per-block quantization), verified by
test-backend-ops MUL_MAT_ID (type_a=nvfp4, broadcast b=1; 790/790 for the
default, gather, and per-expert paths) and by coherent end-to-end
generation. Set GGML_CUDA_NO_MOE_QUANT_DEDUP=1 to force the original
per-expert path.

Same-binary A/B on RTX 5090 (sm_120), Qwen3.6-35B-A3B-NVFP4 prefill @8192
(nsys, graphs-off; the unchanged mul_mat_q GEMM confirms stable clocks):
activation-quant GPU-busy drops 61% (78.2 -> 30.4 ms) with the fused
quantize+scatter, vs 33% (78.2 -> 52.8 ms) for the two-kernel gather. The
fused path avoids materializing and re-reading the 8x compact buffer,
writing the expert copies directly from registers.

* CUDA: bounds-check token ids in build_tok2c_kernel

Guard against malformed ids_src1: skip out-of-range token ids (t < 0 or
t >= n_tokens) and drop entries beyond n_expert_used per token instead of
writing past the token's tok2c region. No behavior change for valid MoE
routing data; test-backend-ops MUL_MAT_ID 790/790.

* Refactor the code based on review comments

- Removed previously added kernels that were not necessary anymore\
- Added an inverse mapping from (token, slot) to compact row. Each token is quantized once and scattered to its compact rows.

* Adding q8_1 support for dedup and addressing review comments

* Add pragma unrolls

* Remove redundant cudaMemsetAsync call

* Removing follow up redundancies

---------

Co-authored-by: praneshgo <227579474+praneshgo@users.noreply.github.com>

ggml/src/ggml-cuda/mmq.cu

@@ -175,13 +175,17 @@ void ggml_cuda_mul_mat_q(
     ggml_cuda_pool_alloc<int32_t> ids_dst(ctx.pool(), ne_get_rows);
     ggml_cuda_pool_alloc<int32_t> expert_bounds(ctx.pool(), ne02 + 1);
 
+    // gate/up activations are broadcast across experts (ne11 == 1): quantize each token once and
+    // scatter to its slots. ids_src1 then holds the inverse map (token slot -> compact row).
+    const bool dedup_bcast = ne11 == 1 && n_expert_used > 1;
+
     {
         GGML_ASSERT(ids->nb[0] == ggml_element_size(ids));
         const int si1  = ids->nb[1] / ggml_element_size(ids);
         const int sis1 = nb12 / nb11;
 
         ggml_cuda_launch_mm_ids_helper((const int32_t *) ids->data, ids_src1.get(), ids_dst.get(), expert_bounds.get(),
-            ne02, ne12, n_expert_used, ne11, si1, sis1, stream);
+            ne02, ne12, n_expert_used, ne11, si1, sis1, /*write_inverse =*/ dedup_bcast, stream);
         CUDA_CHECK(cudaGetLastError());
     }
 
@@ -198,7 +202,16 @@ void ggml_cuda_mul_mat_q(
         const int64_t s12 = src1->nb[2] / ts_src1;
         const int64_t s13 = src1->nb[3] / ts_src1;
 
-        if (use_native_fp4) {
+        if (dedup_bcast) {
+            // quantize each token once, scatter its block to all n_expert_used slots
+            if (use_native_fp4) {
+                quantize_scatter_mmq_fp4_cuda(src1_d, ids_src1.get(), src1_q8_1.get(), src0->type, ne10,
+                                        /*stride_token=*/s12, ne10_padded, ne12, ne11_flat, n_expert_used, stream);
+            } else {
+                quantize_scatter_mmq_q8_1_cuda(src1_d, ids_src1.get(), src1_q8_1.get(), src0->type, ne10,
+                                        /*stride_token=*/s12, ne10_padded, ne12, ne11_flat, n_expert_used, stream);
+            }
+        } else if (use_native_fp4) {
             quantize_mmq_fp4_cuda(src1_d, ids_src1.get(), src1_q8_1.get(), src0->type, ne10, s11, s12, s13,
                                     ne10_padded, ne11_flat, ne12_flat, ne13_flat, stream);
         } else {