Fix error in warn_treedepth when using multiple NUTS samplers

[ricardoV94]

Description

Fix issue reported in https://discourse.pymc.io/t/valueerror-the-truth-value-of-an-array-with-more-than-one-element-is-ambiguous-given-at-the-end-of-sampling/13875/3

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

---

📚 Documentation preview 📚: https://pymc--7182.org.readthedocs.build/en/7182/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.24%. Comparing base (6c6fd13) to head (6a12657).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7182   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         100      100           
  Lines       16887    16887           
=======================================
  Hits        15577    15577           
  Misses       1310     1310           

Files	Coverage Δ
pymc/stats/convergence.py	97.43% <100.00%> (ø)
pymc/step_methods/hmc/nuts.py	97.27% <ø> (ø)

[ricardoV94]

Thanks for the review @junpenglao! However, I'm slightly disappointed you didn't say "LGTM"