ENH speed up beam search

[MatthewSZhang]

Checklist

• Used a personal fork to propose changes
• A reference to a related issue: ENH Speed up beam search #197
• A description of the changes: Use numpy array operators to speed up beam search

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (509ee9c) to head (d93ccf7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #198   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          689       681    -8     
=========================================
- Hits           689       681    -8     

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