Fixed topk op (#805)

* Support bigbird model

* Support GPT2

* rm useless code

* deal with comments

* fixed topk

* deal with comments

* add notes

x2paddle/op_mapper/onnx2paddle/opset9/opset.py

@@ -2577,27 +2577,42 @@ class OpSet9():
     def TopK(self, node):
         val_x = self.graph.get_input_node(node, idx=0, copy=True)
         val_k = self.graph.get_input_node(node, idx=1, copy=True)
-        if val_k.dtype != "int32":
-            self.paddle_graph.add_layer(
-                "paddle.cast",
-                inputs={"x": val_k.name},
-                outputs=[val_k.name],
-                dtype=string('int32'))
         layer_attrs = dict()
         layer_attrs["axis"] = node.get_attr('axis', -1)
         layer_attrs["largest"] = True if node.get_attr('largest',
                                                        1) == 1 else False
         layer_attrs["sorted"] = True if node.get_attr('sorted',
                                                       1) == 1 else False
-        self.paddle_graph.add_layer(
-            "paddle.topk",
-            inputs={"x": val_x.name,
-                    "k": val_k.name},
-            outputs=[
-                "{}_p{}".format(node.layer_name, 0),
-                "{}_p{}".format(node.layer_name, 1)
-            ],
-            **layer_attrs)
+        k = _const_weight_or_none(val_k)
+        if isinstance(k, (list, tuple, np.ndarray)):
+            k = k[0]
+        # If k can get the value directly, it is used as an attribute; otherwise it is used as an input tensor
+        if k is not None:
+            layer_attrs["k"] = k
+            self.paddle_graph.add_layer(
+                "paddle.topk",
+                inputs={"x": val_x.name},
+                outputs=[
+                    "{}_p{}".format(node.layer_name, 0),
+                    "{}_p{}".format(node.layer_name, 1)
+                ],
+                **layer_attrs)
+        else:
+            if val_k.dtype != "int32":
+                self.paddle_graph.add_layer(
+                    "paddle.cast",
+                    inputs={"x": val_k.name},
+                    outputs=[val_k.name],
+                    dtype=string('int32'))
+            self.paddle_graph.add_layer(
+                "paddle.topk",
+                inputs={"x": val_x.name,
+                        "k": val_k.name},
+                outputs=[
+                    "{}_p{}".format(node.layer_name, 0),
+                    "{}_p{}".format(node.layer_name, 1)
+                ],
+                **layer_attrs)
 
     @print_mapping_info
     def LRN(self, node):