Expose maxout Python API. (#11278)

* Expose maxout API.

* Fix code style.

python/paddle/fluid/layers/ops.py

@@ -73,6 +73,7 @@ __all__ = [
     'sum',
     'polygon_box_transform',
     'shape',
+    'maxout',
 ] + __activations__
 
 for _OP in set(__all__):

python/paddle/fluid/tests/unittests/test_layers.py

@@ -387,6 +387,14 @@ class TestBook(unittest.TestCase):
             self.assertIsNotNone(output)
         print(str(program))
 
+    def test_maxout(self):
+        program = Program()
+        with program_guard(program):
+            data = layers.data(name='x', shape=[8, 6, 6], dtype="float32")
+            output = layers.maxout(x=data, groups=2)
+            self.assertIsNotNone(output)
+        print(str(program))
+
 
 if __name__ == '__main__':
     unittest.main()