reverse order of for loop in ggml_build_backward_expand to save memory when using gradient checkpointing and allocator

with this loop order gradient checkpointing with allocator on 16 layer model saves 13% memory; 2 layer memory it saves 2% memory.

the computation results are the same

ggml.c

@@ -15809,7 +15809,7 @@ void ggml_build_backward_expand(struct ggml_context * ctx, struct ggml_cgraph *
         }
     }
 
-    for (int i = gf->n_nodes - 1; i >= 0; i--) {
+    for (int i = 0; i < gf->n_nodes; i++) {
         struct ggml_tensor * node = gf->nodes[i];
 
         if (node->is_param) {