Add build in ModuleBase

Author: gpengzhi

texar/tf/module_base.py

@@ -58,6 +58,12 @@ def call(self, inputs, *args, **kwargs):
         """
         raise NotImplementedError
 
+    def build(self, input_shape):
+        r"""Creates the variables of the layer (optional, for subclass
+        implementers).
+        """
+        super().build(input_shape)
+
     @staticmethod
     def default_hparams():
         r"""Returns a `dict` of hyperparameters of the module with default