Fix RougeL/RougeLSum implementation

[stancld]

What does this PR do?

Fixes #904

This PR fixes RougeL calculation for summaries.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #944 (c174ad6) into master (717622e) will increase coverage by 24%.
The diff coverage is 98%.

@@           Coverage Diff           @@
##           master   #944     +/-   ##
=======================================
+ Coverage      71%    95%    +24%     
=======================================
  Files         173    173             
  Lines        7332   7381     +49     
=======================================
+ Hits         5181   7000   +1819     
+ Misses       2151    381   -1770     

[Borda]

lgtm