Fix SkipLayerNormFusion incorrectly setting modified every time it ru…

…ns (#21502)

### Description
<!-- Describe your changes. -->
Current behavior forces all L2 optimizers to loop until they hit the max
number of iterations.

Only update modified if the graph was modified.

### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Fix unnecessary loops of L2 optimizers during model loading.

onnxruntime/core/optimizer/skip_layer_norm_fusion.cc

@@ -168,7 +168,8 @@ Note: This fusion doesn't consider the following case:
      LayerNormalization
 */
 
-Status SkipLayerNormFusion::ApplyImpl(Graph& graph, bool& modified, int graph_level, const logging::Logger& logger) const {
+Status SkipLayerNormFusion::ApplyImpl(Graph& graph, bool& modified, int graph_level,
+                                      const logging::Logger& logger) const {
   GraphViewer graph_viewer(graph);
   const auto& node_topology_list = graph_viewer.GetNodesInTopologicalOrder();
   InlinedVector<std::reference_wrapper<Node>> nodes_to_remove;
@@ -299,12 +300,15 @@ Status SkipLayerNormFusion::ApplyImpl(Graph& graph, bool& modified, int graph_le
     // Assign provider to this new node. Provider should be same as the provider for old node.
     skip_layer_norm_node.SetExecutionProviderType(ln_node.GetExecutionProviderType());
   }
+
   for (const auto& node : nodes_to_remove) {
     graph_utils::RemoveNodeOutputEdges(graph, node);
     graph.RemoveNode(node.get().Index());
   }
 
-  modified = true;
+  if (!nodes_to_remove.empty()) {
+    modified = true;
+  }
 
   return Status::OK();
 }