Fix repr for custom distributions, add optional constraint

[martinkim0]

Fixes #2392

NegativeBinomial and ZeroInflatedNegativeBinomial would raise an ambiguous ValueError when the following conditions were met:

• scale not passed in and thus defaulted to None
• log_prob called on a tensor not within support

This caused an error on this line when calling __repr__ since self.scale evaluated to None.

Another issue was that validate_args could not be True when any optional parameters are left as None. Fixed with an optional constraint wrapper.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (f0ef18d) 89.20% compared to head (f8699f7) 89.17%.

Files	Patch %	Lines
scvi/distributions/_negative_binomial.py	76.92%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2395      +/-   ##
==========================================
- Coverage   89.20%   89.17%   -0.03%     
==========================================
  Files         152      153       +1     
  Lines       12492    12518      +26     
==========================================
+ Hits        11143    11163      +20     
- Misses       1349     1355       +6     

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