ggml: add ggml_can_fuse_subgraph (#16662)

* ggml: add ggml_can_fuse_subgraph

* ggml-cuda: use ggml_can_fuse_subgraph for topk-moe

* format

* 1. remove inputs from signature as they are transient nodes
2. add check for views: view_src should be part of the subgraph

* - combine check into one loop
- check all view_src parents
- other minor review comments

* remove redudant if test

* - rename and other minor review comments

* add assert about count < 32

ggml/src/ggml.c

@@ -6964,6 +6964,78 @@ void ggml_graph_print(const struct ggml_cgraph * cgraph) {
     GGML_LOG_INFO("========================================\n");
 }
 
+static int ggml_node_list_find_tensor(const struct ggml_cgraph * cgraph,
+                                      const int *                idxs,
+                                      int                        count,
+                                      const struct ggml_tensor * tensor) {
+    GGML_ASSERT(cgraph && idxs);
+    for (int i = 0; i < count; ++i) {
+        const int node_idx = idxs[i];
+
+        if (node_idx >= cgraph->n_nodes) {
+            return -1;
+        }
+        if (cgraph->nodes[node_idx] == tensor) {
+            return i;
+        }
+    }
+    return -1;
+}
+
+bool ggml_can_fuse_subgraph_ext(const struct ggml_cgraph * cgraph,
+                                const int *                node_idxs,
+                                int                        count,
+                                const enum ggml_op *       ops,
+                                const int *                outputs,
+                                int                        num_outputs) {
+    GGML_ASSERT(outputs && num_outputs > 0);
+
+    for (int i = 0; i < count; ++i) {
+        if (node_idxs[i] >= cgraph->n_nodes) {
+            return false;
+        }
+
+        const struct ggml_tensor * node = cgraph->nodes[node_idxs[i]];
+
+        if (node->op != ops[i]) {
+            return false;
+        }
+
+        if (ggml_node_list_find_tensor(cgraph, outputs, num_outputs, node) != -1) {
+            continue;
+        }
+
+        if (node->flags & GGML_TENSOR_FLAG_OUTPUT) {
+            return false;
+        }
+
+        int subgraph_uses = 0;
+        for (int j = i + 1; j < count; ++j) {
+            const struct ggml_tensor * other_node = cgraph->nodes[node_idxs[j]];
+            for (int src_idx = 0; src_idx < GGML_MAX_SRC; src_idx++) {
+                if (other_node->src[src_idx] == node) {
+                    subgraph_uses++;
+                }
+            }
+        }
+
+        if (subgraph_uses != ggml_node_get_use_count(cgraph, node_idxs[i])) {
+            return false;
+        }
+
+        // if node is a view, check if the view_src and all it's parent view_srcs are within the subgraph
+        struct ggml_tensor * view_src = node->view_src;
+        while (view_src) {
+            if (ggml_node_list_find_tensor(cgraph, node_idxs, count, view_src) == -1) {
+                return false;
+            }
+            view_src = view_src->view_src;
+        }
+    }
+
+    return true;
+}
+
 // check if node is part of the graph
 static bool ggml_graph_find(const struct ggml_cgraph * cgraph, const struct ggml_tensor * node) {
     if (cgraph == NULL) {