Fast nb when data known to be sparse

[adamgayoso]

Fixes #1038

[codecov[bot]]

Codecov Report

Merging #1052 (9456145) into master (709d447) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1052      +/-   ##
==========================================
+ Coverage   90.69%   90.71%   +0.01%     
==========================================
  Files          96       96              
  Lines        7761     7772      +11     
==========================================
+ Hits         7039     7050      +11     
  Misses        722      722              

Impacted Files	Coverage Δ
scvi/distributions/_negative_binomial.py	87.93% <100.00%> (+0.81%)	:arrow_up:

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