[cherry-pick]fix multihead matmul shared params (#27321)

* fix multihead matmul shared params (#27121)

* fix multihead matmul shared params

paddle/fluid/framework/ir/multihead_matmul_fuse_pass.cc

@@ -615,9 +615,17 @@ static int BuildFusionV2(Graph* graph, const std::string& name_scope,
     GET_IR_NODE_FROM_SUBGRAPH(transpose2_qkv_out, transpose2_qkv_out,
                               multihead_pattern);
 
-    fuse_creater(input0, mul0, mul1, mul2, mul0_out, mul1_out, mul2_out, mul0_w,
-                 mul1_w, mul2_w, eltadd0_b, eltadd1_b, eltadd2_b, eltadd_qk_b,
-                 reshape2_0, reshape2_qkv_out, scale, scale_out);
+    // If weights or biases in qkv's fc are shared by multiple multihead_matmul
+    // patterns, we do not support this kind of fusion, this pass will not take
+    // effect.
+    bool is_fc_params_shared =
+        mul0_w->outputs.size() > 1 || mul1_w->outputs.size() > 1 ||
+        mul2_w->outputs.size() > 1 || eltadd0_b->outputs.size() > 1 ||
+        eltadd1_b->outputs.size() > 1 || eltadd2_b->outputs.size() > 1;
+    if (!is_fc_params_shared) {
+      fuse_creater(input0, mul0, mul1, mul2, mul0_out, mul1_out, mul2_out,
+                   mul0_w, mul1_w, mul2_w, eltadd0_b, eltadd1_b, eltadd2_b,
+                   eltadd_qk_b, reshape2_0, reshape2_qkv_out, scale, scale_out);
 
       std::unordered_set<const Node*> marked_nodes({eltadd0,
                                                     eltadd1,
@@ -662,6 +670,7 @@ static int BuildFusionV2(Graph* graph, const std::string& name_scope,
       // Remove unneeded nodes.
       GraphSafeRemoveNodes(graph, marked_nodes);
       ++fusion_count;
+    }
   };
   gpd(graph, handler);