code: def _pack(cls, tensors):
      
      if not tensors:
        return None
      elif len(tensors) == 1:
        return array_ops.reshape(tensors[0], [-1])
      else:
        flattened = [array_ops.reshape(tensor, [-1]) for tensor in tensors]
        return array_ops.concat(flattened, 0)
docs: Pack a list of `Tensor`s into a single, flattened, rank-1 `Tensor`.