Update developer_guide_implementing_distribution.md

[mjhajharia]

added an import

[ricardoV94]

Thanks!

[codecov]

Codecov Report

Merging #4889 (4b44de4) into main (819f045) will decrease coverage by 10.79%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main    #4889       +/-   ##
===========================================
- Coverage   73.16%   62.37%   -10.80%     
===========================================
  Files          86       86               
  Lines       13838    13838               
===========================================
- Hits        10125     8631     -1494     
- Misses       3713     5207     +1494     

Impacted Files	Coverage Δ
pymc3/ode/utils.py	17.85% <0.00%> (-82.15%)	⬇️
pymc3/step_methods/mlda.py	11.94% <0.00%> (-74.55%)	⬇️
pymc3/gp/cov.py	29.20% <0.00%> (-68.88%)	⬇️
pymc3/ode/ode.py	24.24% <0.00%> (-60.61%)	⬇️
pymc3/step_methods/hmc/hmc.py	33.33% <0.00%> (-58.83%)	⬇️
pymc3/step_methods/metropolis.py	35.49% <0.00%> (-47.77%)	⬇️
pymc3/tests/models.py	40.28% <0.00%> (-47.49%)	⬇️
pymc3/gp/mean.py	51.28% <0.00%> (-46.16%)	⬇️
pymc3/gp/util.py	37.14% <0.00%> (-42.86%)	⬇️
pymc3/gp/gp.py	17.86% <0.00%> (-27.05%)	⬇️
... and 23 more

[michaelosthege]

CI failures are unrelated