FIX: Invariant input_spec for WeightNormalization (#687)

* Make the first dimension `None` to support invariant batch size.
* Add test case to check compatibility of WeightNormalization with
  TimeDistributed.

tensorflow_addons/layers/wrappers.py

@@ -62,7 +62,8 @@ def __init__(self, layer, data_init=True, **kwargs):
     def build(self, input_shape):
         """Build `Layer`"""
         input_shape = tf.TensorShape(input_shape).as_list()
-        self.input_spec = tf.keras.layers.InputSpec(shape=input_shape)
+        self.input_spec = tf.keras.layers.InputSpec(
+            shape=[None] + input_shape[1:])
 
         if not self.layer.built:
             self.layer.build(input_shape)

tensorflow_addons/layers/wrappers_test.py

@@ -73,6 +73,14 @@ def test_weightnorm_non_kernel_layer(self):
             wn_wrapper = wrappers.WeightNormalization(non_kernel_layer)
             wn_wrapper(images)
 
+    def test_weightnorm_with_time_dist(self):
+        batch_shape = (32, 16, 64, 64, 3)
+        inputs = tf.keras.layers.Input(batch_shape=batch_shape)
+        a = tf.keras.layers.Conv2D(3, 5)
+        b = wrappers.WeightNormalization(a)
+        out = tf.keras.layers.TimeDistributed(b)(inputs)
+        model = tf.keras.Model(inputs, out)
+
 
 if __name__ == "__main__":
     tf.test.main()