Improve function is_control_flow_to_transform. test=develop (#23109)

python/paddle/fluid/dygraph/dygraph_to_static/utils.py

@@ -93,8 +93,10 @@ def is_control_flow_to_transform(node, var_name_to_type):
         "The type of input node must be gast.AST, but received %s." % type(node)
 
     if isinstance(node, gast.If):
-        # TODO: make a better condition
-        return True
+        from .ifelse_transformer import IfConditionVisitor
+        if_visitor = IfConditionVisitor(
+            node.test, node_var_type_map=var_name_to_type)
+        return if_visitor.is_control_flow()
 
     if isinstance(node, gast.For):
         # TODO: make a better condition

python/paddle/fluid/tests/unittests/dygraph_to_static/test_list.py

@@ -27,7 +27,9 @@ def test_list_without_control_flow(x):
     # Python list will not be transformed.
     x = fluid.dygraph.to_variable(x)
     a = []
-    a.append(x)
+    # It's a plain python control flow which won't be transformed
+    if 2 > 1:
+        a.append(x)
     return a