Determines the approximate scores for translating every subsequence in src_sentence

Future scores can be used a look-ahead to determine the difficulty of translating the remaining parts of a src_sentence.

Returns:Scores of subsequences referenced by their start and

end positions. For example, result[2][5] is the score of the subsequence covering positions 2, 3, and 4. :rtype: dict(int: (dict(int): float))