Fix nested sequential deferred build (#10655)

* Fix nested sequential deferred build

* Add test

* Unify style

keras/engine/sequential.py

@@ -149,8 +149,6 @@ class Sequential(Model):
                     first_layer = layer.layers[0]
                     while isinstance(first_layer, (Model, Sequential)):
                         first_layer = first_layer.layers[0]
-                    batch_shape = first_layer.batch_input_shape
-                    dtype = first_layer.dtype
 
                 if hasattr(first_layer, 'batch_input_shape'):
                     batch_shape = first_layer.batch_input_shape

tests/keras/test_sequential_model.py

@@ -422,5 +422,45 @@ def test_sequential_deferred_build():
     assert len(new_model.weights) == 4
 
 
+@keras_test
+def test_nested_sequential_deferred_build():
+    inner_model = keras.models.Sequential()
+    inner_model.add(keras.layers.Dense(3))
+    inner_model.add(keras.layers.Dense(3))
+
+    model = keras.models.Sequential()
+    model.add(inner_model)
+    model.add(keras.layers.Dense(5))
+    model.compile('sgd', 'mse')
+
+    assert inner_model.built is False
+    assert len(inner_model.layers) == 2
+    assert len(inner_model.weights) == 0
+    assert model.built is False
+    assert len(model.layers) == 2
+    assert len(model.weights) == 0
+
+    model.train_on_batch(
+        np.random.random((2, 4)), np.random.random((2, 5)))
+
+    assert inner_model.built is True
+    assert len(inner_model.layers) == 2
+    assert len(inner_model.weights) == 4
+    assert model.built is True
+    assert len(model.layers) == 2
+    assert len(model.weights) == 6
+
+    config = model.get_config()
+    new_model = keras.models.Sequential.from_config(config)
+    assert new_model.built is True
+    assert len(new_model.layers) == 2
+    assert len(new_model.weights) == 6
+
+    new_inner_model = new_model.layers[0]
+    assert new_inner_model.built is True
+    assert len(new_inner_model.layers) == 2
+    assert len(new_inner_model.weights) == 4
+
+
 if __name__ == '__main__':
     pytest.main([__file__])