Pack a list of `Tensor`s into a single, flattened, rank-1 `Tensor`.

def _pack(cls, tensors):
    """Pack a list of `Tensor`s into a single, flattened, rank-1 `Tensor`."""
    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)