[Bug] Fixed ONNX Gemm bug (#917)

* fixed Gemm bug

* re-lint

x2paddle/op_mapper/onnx2paddle/opset_legacy.py

@@ -1637,16 +1637,57 @@ class OpSet():
             "transpose_x": trans_a,
             "transpose_y": trans_b,
         }
+        if abs(alpha - 1.0) < 1e-5:
+            if abs(beta - 0.0) < 1e-5:
+                self.paddle_graph.add_layer(
+                    'paddle.matmul',
+                    inputs=matmul_inputs,
+                    outputs=[node.name],
+                    **attr_matmul)
+            else:
                 self.paddle_graph.add_layer(
                     'paddle.matmul',
                     inputs=matmul_inputs,
                     outputs=[val_mm],
                     **attr_matmul)
+                if abs(beta - 1.0) < 1e-5:
+                    add_inputs = {"x": val_mm, "y": val_c.name}
                     self.paddle_graph.add_layer(
-            "paddle.scale", inputs={"x": val_mm}, outputs=[val_mm], scale=alpha)
-
-        if beta != 0:
-            if beta == 1.:
+                        "paddle.add", inputs=add_inputs, outputs=[node.name])
+                else:
+                    var_beta = node.name + '_beta'
+                    self.paddle_graph.add_layer(
+                        "paddle.scale",
+                        inputs={"x": val_c.name},
+                        outputs=[var_beta],
+                        scale=beta)
+                    add_inputs = {"x": val_mm, "y": var_beta}
+                    self.paddle_graph.add_layer(
+                        "paddle.add", inputs=add_inputs, outputs=[node.name])
+        else:
+            if abs(beta - 0.0) < 1e-5:
+                self.paddle_graph.add_layer(
+                    'paddle.matmul',
+                    inputs=matmul_inputs,
+                    outputs=[val_mm],
+                    **attr_matmul)
+                self.paddle_graph.add_layer(
+                    "paddle.scale",
+                    inputs={"x": val_mm},
+                    outputs=[node.name],
+                    scale=alpha)
+            else:
+                self.paddle_graph.add_layer(
+                    'paddle.matmul',
+                    inputs=[matmul_inputs],
+                    outputs=[val_mm],
+                    **attr_matmul)
+                self.paddle_graph.add_layer(
+                    "paddle.scale",
+                    inputs={"x": val_mm},
+                    outputs=[val_mm],
+                    scale=alpha)
+                if abs(beta - 1.0) < 1e-5:
                     add_inputs = {"x": val_mm, "y": val_c.name}
                     self.paddle_graph.add_layer(
                         "paddle.add", inputs=add_inputs, outputs=[node.name])