Fix that convert_var_shape doesn't support slice like [0:], test=develop (#31051)

As the title, when slice_node like 1:3 being passed to idx of convert_var_shape, it will cause syntax error because a function cannot take this as argument. This PR fixed it.

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

@@ -31,13 +31,20 @@ def create_convert_shape_node(var_shape_node,
 
     if isinstance(var_shape_node, gast.Attribute):
         args = [ast_to_source_code(var_shape_node.value).strip()]
-        if slice_node:
+        # (1) A slice can be a simple number such as 1, -2, i.e. gast.Index
+        # (2) A slice can also be represented by bounds such as 2:-1, i.e. not gast.Index
+        # In (1) case, we pass the number as 'idx' argument in convert_var_shape
+        # In (2) case, we have to make it like `convert_var_shape(x)[slice]`
+        if slice_node is not None and isinstance(slice_node, gast.Index):
             args.append(ast_to_source_code(slice_node).strip())
 
         convert_var_shape_func = "paddle.jit.dy2static.convert_var_shape({}, in_control_flow={})".format(
             ",".join(args), in_control_flow)
-
         api_shape_node = gast.parse(convert_var_shape_func).body[0].value
+
+        if slice_node is not None and not isinstance(slice_node, gast.Index):
+            return gast.Subscript(
+                value=api_shape_node, slice=slice_node, ctx=gast.Load())
         return api_shape_node
 
     if isinstance(var_shape_node, gast.Subscript):

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

@@ -60,6 +60,16 @@ def dyfunc_tensor_shape_5(x):
     return res
 
 
+def dyfunc_tensor_shape_6(x):
+    # `res = fluid.layers.reshape(x, shape=(-1, s))` to
+    # `res = fluid.layers.reshape(x, shape=(-1,
+    #           paddle.jit.dy2static.convert_var_shape(x)[0:]))`
+    x = fluid.dygraph.to_variable(x)
+    s = x.shape[0:]
+    res = fluid.layers.reshape(x, shape=s)
+    return res
+
+
 def dyfunc_tuple_shape_1(x):
     x = paddle.to_tensor(x)
     a, b = x.shape
@@ -280,6 +290,11 @@ class TestTensorShapeBasic5(TestTensorShapeBasic):
         self.dygraph_func = dyfunc_tensor_shape_5
 
 
+class TestTensorShapeBasic6(TestTensorShapeBasic):
+    def init_test_func(self):
+        self.dygraph_func = dyfunc_tensor_shape_6
+
+
 class TestTupleShape1(TestTensorShapeBasic):
     def init_test_func(self):
         self.input = numpy.ones((5, 7)).astype("int32")