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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add(self, watch_key, tensor_value):
 """Add a tensor value. Args: watch_key: A string representing the debugger tensor watch, e.g., 'Dense_1/BiasAdd:0:DebugIdentity'. tensor_value: The value of the tensor as a numpy.ndarray. """

    if watch_key not in self._tensor_data:
      self._tensor_data[watch_key] = _WatchStore(
          watch_key,
          mem_bytes_limit=self._watch_mem_bytes_limit)
    self._tensor_data[watch_key].add(tensor_value)