[AnomalyDetection] Support offline detectors

[shunping]

Depends on #34310

[codecov]

Codecov Report

Attention: Patch coverage is 92.59259% with 8 lines in your changes missing coverage. Please review.

Project coverage is 56.36%. Comparing base (5950a17) to head (57a3381).
Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/ml/anomaly/specifiable.py	93.65%	4 Missing ⚠️
...python/apache_beam/ml/anomaly/detectors/offline.py	88.88%	2 Missing ⚠️
sdks/python/apache_beam/ml/anomaly/transforms.py	92.30%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #34311      +/-   ##
============================================
+ Coverage     56.34%   56.36%   +0.02%     
  Complexity     3286     3286              
============================================
  Files          1173     1174       +1     
  Lines        179037   179169     +132     
  Branches       3398     3398              
============================================
+ Hits         100877   100995     +118     
- Misses        74903    74917      +14     
  Partials       3257     3257              

Flag	Coverage Δ
python	81.33% <92.59%> (+0.01%)	⬆️

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[shunping]

r: @damccorm

[github-actions]

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[damccorm]

This generally LGTM, just had a minor comment. I'll wait on #34311 before merging anyways though (I didn't review the specifiable pieces of this PR since that is part of that PR).

[damccorm]

Suggested change

	custom_detector: The `OfflineDetector` to run.
	offline_detector: The `OfflineDetector` to run.

[damccorm]

Thanks!