#174 fixed that unk_replacement and beam_search work together

[nem6ishi]

The issue that unk_replacement and beam_search does not work is solved in this pull request.

The problem is that there is no attension_scores but multiple attension_scores in beam_search which is named "beam_search_output.original_outputs.attention_scores".
So I just added the procedures to pick one of them.
Since it seems that the top of the list is always the best among the beams in beam_search, [:,0,:] of it was taken.

This is my first big pull request and the code does not look beautiful, but I will appreciate if I could get kind instructions for this.

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[nem6ishi]

I signed it!

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