docs: add custom model runner example

[parano]

What does this PR address?

Fixes #(issue)

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[pep8speaks]

Hello @parano, Thanks for updating this PR.

There are currently no PEP 8 issues detected in this PR. Cheers! 🍻

Comment last updated at 2022-08-09 12:08:18 UTC

[codecov]

Codecov Report

Merging #2885 (4acddb5) into main (0477d10) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2885      +/-   ##
==========================================
- Coverage   70.72%   70.70%   -0.02%     
==========================================
  Files         104      103       -1     
  Lines        9332     9302      -30     
==========================================
- Hits         6600     6577      -23     
+ Misses       2732     2725       -7     

Impacted Files	Coverage Δ
bentoml/_internal/utils/click.py

[aarnphm]

minor details.