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-impl.h

@@ -647,6 +647,36 @@ static inline bool ggml_can_fuse(const struct ggml_cgraph * cgraph, int node_idx
     return ggml_can_fuse_ext(cgraph, idxs, ops, num_ops);
 }
 
+GGML_API 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);
+
+// Returns true if the subgraph formed by {node_idxs} can be fused
+// checks whethers all nodes which are not part of outputs can be elided
+// by checking if their num_uses are confined to the subgraph
+static inline bool ggml_can_fuse_subgraph(const struct ggml_cgraph * cgraph,
+                                          int                        node_idx,
+                                          int                        count,
+                                          const enum ggml_op *       ops,
+                                          const int *                outputs,
+                                          int                        num_outputs) {
+    GGML_ASSERT(count < 32);
+    if (node_idx + count > cgraph->n_nodes) {
+        return false;
+    }
+
+    int idxs[32];
+
+    for (int i = 0; i < count; ++i) {
+        idxs[i] = node_idx + i;
+    }
+
+    return ggml_can_fuse_subgraph_ext(cgraph, idxs, count, ops, outputs, num_outputs);
+}
+
 #ifdef __cplusplus
 }
 #endif
@@ -660,6 +690,13 @@ inline bool ggml_can_fuse(const struct ggml_cgraph * cgraph, int node_idx, std::
     return ggml_can_fuse(cgraph, node_idx, ops.begin(), (int)ops.size());
 }
 
+inline bool ggml_can_fuse_subgraph(const struct ggml_cgraph *          cgraph,
+                                   int                                 start_idx,
+                                   std::initializer_list<enum ggml_op> ops,
+                                   std::initializer_list<int>          outputs = {}) {
+    return ggml_can_fuse_subgraph(cgraph, start_idx, ops.size(), ops.begin(), outputs.begin(), outputs.size());
+}
+
 // expose GGUF internals for test code
 GGML_API size_t gguf_type_size(enum gguf_type type);
 GGML_API struct gguf_context * gguf_init_from_file_impl(FILE * file, struct gguf_init_params params);