fix the bug under third_party/py/lingvo/tasks/mt/input_generator.py that if self.params.mass_layer is None, self.mass_layer does not exist and complains 'mass_layer' lookup failure.
fix the bug under third_party/py/lingvo/tasks/mt/input_generator.py that if self.params.mass_layer is None, self.mass_layer does not exist and complains 'mass_layer' lookup failure.

PiperOrigin-RevId: 439954363

tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo