* Added wrapper for gamma sampling in negative binomial distribution.

* Fixed typo in posterior sampling warnings.

* Drop nuts init method from pm.sample (#3863)

* Dropped nuts init method

* Dropped nuts init method from tests

* Refined doc string and added release note

* Added wrapper for gamma sampling in negative binomial distribution.

* Fixed typo in posterior sampling warnings.

* Added test for negative binomial input shape.

* Updating release notes with changes to negative binomial sampling.

Co-authored-by: Alexandre ANDORRA <andorra.alexandre@gmail.com>