NotImplementedError stack trace for model.fit() with tf.distribute.MirroredStrategy()

stack trace

---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-26-55bddb4c82a6> in <module>
      1 # dist_model.summary()
----> 2 history = dist_model.fit(trainx, trainy_hot, epochs=1, validation_data = (testx, testy_hot),batch_size=64, verbose=1)

~\Anaconda3\envs\open_cv\lib\site-packages\keras\engine\training.py in fit(self, x, y, batch_size, epochs, verbose, callbacks, validation_split, validation_data, shuffle, class_weight, sample_weight, initial_epoch, steps_per_epoch, validation_steps, validation_freq, max_queue_size, workers, use_multiprocessing, **kwargs)
   1211         else:
   1212             fit_inputs = x + y + sample_weights
-> 1213         self._make_train_function()
   1214         fit_function = self.train_function
   1215 

~\Anaconda3\envs\open_cv\lib\site-packages\keras\engine\training.py in _make_train_function(self)
    314                     training_updates = self.optimizer.get_updates(
    315                         params=self._collected_trainable_weights,
--> 316                         loss=self.total_loss)
    317                 updates = self.updates + training_updates
    318 

~\Anaconda3\envs\open_cv\lib\site-packages\keras\legacy\interfaces.py in wrapper(*args, **kwargs)
     89                 warnings.warn('Update your `' + object_name + '` call to the ' +
     90                               'Keras 2 API: ' + signature, stacklevel=2)
---> 91             return func(*args, **kwargs)
     92         wrapper._original_function = func
     93         return wrapper

~\Anaconda3\envs\open_cv\lib\site-packages\keras\backend\tensorflow_backend.py in symbolic_fn_wrapper(*args, **kwargs)
     73         if _SYMBOLIC_SCOPE.value:
     74             with get_graph().as_default():
---> 75                 return func(*args, **kwargs)
     76         else:
     77             return func(*args, **kwargs)

~\Anaconda3\envs\open_cv\lib\site-packages\keras\optimizers.py in get_updates(self, loss, params)
    548 
    549             # Apply constraints.
--> 550             if getattr(p, 'constraint', None) is not None:
    551                 new_p = p.constraint(new_p)
    552 

~\Anaconda3\envs\open_cv\lib\site-packages\tensorflow_core\python\ops\variables.py in constraint(self)
    569       Can be `None` if no constraint was passed.
    570     """
--> 571     raise NotImplementedError
    572 
    573   def assign(self, value, use_locking=False, name=None, read_value=True):

NotImplementedError: