Incorrect Doc for VIAlgorithm Class

[gil2rok]

The VIAlgorithm class has code and documentation that do not match in base.py here.

The documentation indicates this class is instantiated with a tuple of approximation and sample functions. However, the code indicates this class is instantiated with a tuple of init, step, and sample functions.

Either the doc or code should change to be consistent. IMO the doc should change as documentation simply describes what the code does.

[gil2rok]

Some VI algorithms take an Optimizer (e.g. mean field, pathfinder) even though the doc does not specify this.