code: def delete(self, key):
      
      self._cur_batch.delete(key)
      self._num_mutations += 1
      if self._num_mutations >= MAX_MUTATIONS_IN_BATCH:
        self.commit()
        self.begin()
docs: Adds deletion of the entity with given key to the mutation buffer.
  
      If mutation buffer reaches its capacity then this method commit all pending
      mutations from the buffer and emties it.
  
      Args:
        key: key of the entity which should be deleted