llamafile : improve moe prompt eval speed on cpu

This change introduces a llamafile_mixmul() API that allows tinyBLAS to
speed up "Mixture of Expert" models. On my Threadripper, Mixtral's 8x7b
F16 weights now process prompts 2x faster. I'm also seeing a 60 percent
improvement with Mixtral 8x22b Q4_0. The same applies to Q8_0, which is
also supported by tinyBLAS. MoE models spend the majority of their time
inside MUL_MAT_ID rather than MUL_MAT, which is why llamafile_sgemm was
not able to help them before. llamafile_mixmul works by decomposing the
mixmul operation into approximatively two sgemm calls.

ggml.c

@@ -11003,11 +11003,14 @@ static void ggml_compute_forward_mul_mat_id(
     const struct ggml_tensor * src1 = dst->src[1];
     const struct ggml_tensor * ids = dst->src[2];
 
-    GGML_TENSOR_BINARY_OP_LOCALS
+    if (llamafile_mixmul(params, src0, src1, ids, dst))
+        return;
 
     const int ith = params->ith;
     const int nth = params->nth;
 
+    GGML_TENSOR_BINARY_OP_LOCALS
+
     const enum ggml_type type = src0->type;
 
     const bool src1_cont = ggml_is_contiguous(src1);
@@ -18504,6 +18507,7 @@ struct ggml_cplan ggml_graph_plan(const struct ggml_cgraph * cgraph, int n_threa
                     cur = 0;
                     const struct ggml_tensor * src0 = node->src[0];
                     const struct ggml_tensor * src1 = node->src[1];
+                    const struct ggml_tensor * src2 = node->src[2];
                     const enum ggml_type vec_dot_type = type_traits[src0->type].vec_dot_type;
                     if (src1->type != vec_dot_type) {
                         cur += ggml_row_size(vec_dot_type, ggml_nelements(src1));
@@ -18512,6 +18516,8 @@ struct ggml_cplan ggml_graph_plan(const struct ggml_cgraph * cgraph, int n_threa
                     cur += GGML_PAD(cur, sizeof(int64_t));       // align
                     cur += n_as * sizeof(int64_t);               // matrix_row_counts
                     cur += n_as * src1->ne[2] * sizeof(int64_t); // matrix_rows
+                    size_t cur2 = llamafile_mixmul_needs(src0, src1, src2);
+                    cur = cur > cur2 ? cur : cur2;
                 } break;
             case GGML_OP_OUT_PROD:
                 {