fix get_likelihood_parameter() bug

[galenxing]

Fixes #874

[codecov]

Codecov Report

Merging #967 (116db7c) into master (d659995) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #967      +/-   ##
==========================================
+ Coverage   89.60%   89.63%   +0.03%     
==========================================
  Files          87       87              
  Lines        6126     6129       +3     
==========================================
+ Hits         5489     5494       +5     
+ Misses        637      635       -2     

Impacted Files	Coverage Δ
scvi/model/base/_rnamixin.py	92.43% <100.00%> (+0.12%)	⬆️
scvi/module/_vae.py	92.85% <0.00%> (+0.95%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d659995...116db7c. Read the comment docs.