[AnomalyDetection] Add base classes and specifiable protocol

[shunping]

This is the first part of the code push for anomaly detection transform.

[shunping]

r: @damccorm

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Codecov Report

Attention: Patch coverage is 91.16022% with 16 lines in your changes missing coverage. Please review.

Project coverage is 59.12%. Comparing base (5aae10d) to head (bc6fa8f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/ml/anomaly/base.py	83.60%	10 Missing ⚠️
sdks/python/apache_beam/ml/anomaly/specifiable.py	95.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #33845      +/-   ##
============================================
+ Coverage     59.08%   59.12%   +0.03%     
  Complexity     3237     3237              
============================================
  Files          1156     1158       +2     
  Lines        176907   177113     +206     
  Branches       3391     3391              
============================================
+ Hits         104532   104715     +183     
- Misses        69008    69031      +23     
  Partials       3367     3367              

Flag	Coverage Δ
python	81.25% <91.16%> (+0.02%)	⬆️

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[damccorm]

Mostly LGTM, but had some questions. In general, docs would be really helpful here. I really like the concept

[damccorm]

I'm having a hard time following this - I think some of it is just indented python is hard to read, but I think it would be very helpful to have a walkthrough here of what this function is doing

I think I've mostly figured out how it is working, but it would help to have this background.

[shunping]

I added some more comments. PTAL.

[damccorm]

Maybe I'm not understanding this well, but could we just do something a bit simpler like:

def new_getattr(self, name):
   if not self._initialized:
      original_init()
      self._initialized = True
   return orig_getattr(self, name)

if we keep track of the original getattr function in orig_getattr

[shunping]

The general idea is like this, but there are some edge cases we need to avoid. Otherwise, we will end up with an infinite loop. I refactor the code a bit to improve its readability. PTAL.

[damccorm]

Why do we start with these registered? Would it be cleaner to just register them on import?

[shunping]

These are the accepted subspaces for the known specifiable, but not the actual specifiable classes. The purpose is to avoid using a global space for every specifiable classes.

I think they should be predefined before the true registration takes place. WDYT?

[shunping]

@damccorm, I've added docstrings and refactored some code for readability. PTAL. Thanks!