[empathetic dialogues] Whitespace changes

[stephenroller]

Patch description
We had a small bug with one label not being in the label candidates causing a crash in the new ED teacher (#4405). Upon inspection, it was because of a trailing whitespace in the candidate but not the text. Upon further inspection, I found the data had lots of trailing whitespaces. This normalizes that.

Testing steps

CI and parlai em -t empathetic_dialogues -m repeat_query/repeat_label

Before #4405:

$ parlai em -t empathetic_dialogues -m repeat_query
    accuracy   bleu-4  exs    f1
    .0001743 .0007299 5738 .1060

$ parlai em -t empathetic_dialogues -m repeat_label
    accuracy  bleu-4  exs    f1
           1   .9645 5738 .9998

After #4405:
(Crashes)

With this PR:

$ parlai em -t empathetic_dialogues -m repeat_query
    accuracy   bleu-4  exs    f1
    .0001743 .0007299 5738 .1060

$ parlai em -t empathetic_dialogues -m repeat_label
    accuracy  bleu-4  exs    f1
           1   .9645 5738 .9998

[EricMichaelSmith]

Wow, would not have expected that, but great find =/ Thanks for the fix!