Add proper error message when user does not call "super.__init__()" in custom layer.

The current error message is quite obscure and surprising.

PiperOrigin-RevId: 290557761
Change-Id: Ibf91099bceb29b562e100e399aaf4757c74060d6

tensorflow/python/keras/engine/base_layer.py

@@ -650,7 +650,11 @@ class Layer(module.Module):
 
     Raises:
       ValueError: if the layer's `call` method returns None (an invalid value).
+      RuntimeError: if `super().__init__()` was not called in the constructor.
     """
+    if not hasattr(self, '_thread_local'):
+      raise RuntimeError(
+          'You must call `super().__init__()` in the layer constructor.')
     call_context = base_layer_utils.call_context()
     input_list = nest.flatten(inputs)
 

tensorflow/python/keras/engine/base_layer_test.py

@@ -582,6 +582,17 @@ class BaseLayerTest(keras_parameterized.TestCase):
     model = keras.Sequential(dense)
     self.assertEqual(model.count_params(), 16 * 4 + 16)
 
+  def test_super_not_called(self):
+
+    class CustomLayerNotCallingSuper(keras.layers.Layer):
+
+      def __init__(self):
+        pass
+
+    layer = CustomLayerNotCallingSuper()
+    with self.assertRaisesRegexp(RuntimeError, 'You must call `super()'):
+      layer(np.random.random((10, 2)))
+
 
 class SymbolicSupportTest(test.TestCase):