fix missing torchtext.Batch import when torchtext not installed

[awaelchli]

What does this PR do?

Follow up to #2395 in which we forgot to add the else clause.
Since torchtext is optional, we can't import torchtext.Batch if it's not installed.

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[Borda]

yeah, I had it there in #2395 :]

[codecov]

Codecov Report

Merging #2402 into master will decrease coverage by 0%.
The diff coverage is 0%.

@@          Coverage Diff           @@
##           master   #2402   +/-   ##
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- Coverage      88%     88%   -0%     
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  Files          69      69           
  Lines        5468    5469    +1     
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  Hits         4834    4834           
- Misses        634     635    +1