Avoid vectorizing num_chain=1

[junpenglao]

Close #317

[kyleabeauchamp]

LGTM, do we want to copy in the MRE from #317 as a test?

[kyleabeauchamp]

I'm not super aware of possible side effects of this change though, so someone else probably needs approve.

[codecov[bot]]

Codecov Report

Merging #318 into master will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #318   +/-   ##
=======================================
  Coverage   90.89%   90.89%           
=======================================
  Files          36       36           
  Lines        2822     2822           
=======================================
  Hits         2565     2565           
  Misses        257      257           

Impacted Files	Coverage Δ
pymc4/mcmc/samplers.py	93.57% <100.00%> (ø)

[junpenglao]

Filed an issue on TFP https://github.com/tensorflow/probability/issues/1071