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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 _gather_topk_beams(nested, score_or_log_prob, batch_size, beam_size):
 """Gather top beams from nested structure."""

  _, topk_indexes = tf.nn.top_k(score_or_log_prob, k=beam_size)
  return _gather_beams(nested, topk_indexes, batch_size, beam_size)