bleuscore: Add weights argument to allow non-uniform n-gram weights

[stancld]

What does this PR do?

This PR adds an argument for BLEUScore and SacreBLEUScore metrics allowing to define own n-gram weights for the score calculation, and thus enables to use different than uniform weights.

Fixes #1074

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1075 (8d8fb19) into master (c04090b) will decrease coverage by 0%.
The diff coverage is 84%.

@@          Coverage Diff           @@
##           master   #1075   +/-   ##
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- Coverage      95%     94%   -0%     
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  Files         181     181           
  Lines        8029    8040   +11     
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- Hits         7601    7597    -4     
- Misses        428     443   +15