Fix the slice for plotting attention

[daniel-vainsencher]

Description

Before the fix, works only when interpretation["attention"] matches encoder_length (in which case the slice is not needed anyway). After, length matches always.

I'm running into this with encoder_length=0, which is probably a bug elsewhere, possibly in my code, but this is probably worth fixing anyway.

Checklist

NOT YET, will do if PR is desired

• Linked issues (if existing)
• Amended changelog for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Make sure to have fun coding!