fix multihead matmul shared params (#27121)

paddle/fluid/framework/ir/multihead_matmul_fuse_pass.cc

@@ -615,6 +615,16 @@ static int BuildFusionV2(Graph* graph, const std::string& name_scope,
     GET_IR_NODE_FROM_SUBGRAPH(transpose2_qkv_out, transpose2_qkv_out,
                               multihead_pattern);
 
+    // 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) {
+      return;
+    }
     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);