code: def mask_coverage(coverage: mx.sym.Symbol, source_length: mx.sym.Symbol) -> mx.sym.Symbol:
      
      return mx.sym.SequenceMask(data=coverage, axis=1, use_sequence_length=True, sequence_length=source_length)
docs: Masks all coverage scores that are outside the actual sequence.
  
      :param coverage: Input coverage vector. Shape: (batch_size, seq_len, coverage_num_hidden).
      :param source_length: Source length. Shape: (batch_size,).
      :return: Masked coverage vector. Shape: (batch_size, seq_len, coverage_num_hidden).