refactor: Do one prediction per input sequence, easier experimentation

[carlosgjs]

The prediction/evaluation interfaces used List[List[str]] as the prediction type to account for: a) batch inference, i.e. generate docs for multiple code snippets in one pass and b) generating multiple predictions per input (sampling). But we've now decided to do deterministic prediction (temp=0) so there's no need for the second level.

This PR changes to the prediction return type to List[str], which also makes the code simpler.

Two additional changes:

• Replace the use of the max_length parameter in favor of max_new_tokens
• Break-up the eval function into eval_promp for easier prompt experimentation
• Updated generate.ipynb notebook accordingly

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (10294bc) 97.00% compared to head (6f2c5da) 97.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
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+ Coverage   97.00%   97.17%   +0.16%     
==========================================
  Files           3        3              
  Lines         167      177      +10     
==========================================
+ Hits          162      172      +10     
  Misses          5        5              

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