[cherry pick]fix a bug of Sequential::__getitem__ (#30899) (#31192)

* fix a bug of Sequential::__getitem__, test=develop

python/paddle/fluid/dygraph/container.py

@@ -67,6 +67,15 @@ class Sequential(Layer):
                 self.add_sublayer(str(idx), layer)
 
     def __getitem__(self, name):
+        if isinstance(name, slice):
+            return self.__class__(*(list(self._sub_layers.values())[name]))
+        else:
+            if name >= len(self._sub_layers):
+                raise IndexError('index {} is out of range'.format(name))
+            elif name < 0 and name >= -len(self._sub_layers):
+                name += len(self._sub_layers)
+            elif name < -len(self._sub_layers):
+                raise IndexError('index {} is out of range'.format(name))
             return self._sub_layers[str(name)]
 
     def __setitem__(self, name, layer):

python/paddle/fluid/tests/test_sequential.py

+#   Copyright (c) 2021 PaddlePaddle Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import unittest
+import paddle
+
+
+class TestDataFeeder(unittest.TestCase):
+    def test_lod_level_1_converter(self):
+        sequential = paddle.nn.Sequential()
+
+        for i in range(10):
+            sequential.add_sublayer(str(i), paddle.nn.Linear(i + 1, i + 1))
+
+        for item in sequential:
+            tmp = item
+
+        tmp = sequential[3:5]
+        self.assertEqual(len(tmp), 2)
+
+        tmp = sequential[-1]
+        self.assertEqual(tmp, sequential[9])
+
+        with self.assertRaises(IndexError):
+            tmp = sequential[10]
+
+        with self.assertRaises(IndexError):
+            tmp = sequential[-11]
+
+
+if __name__ == '__main__':
+    unittest.main()