Implicit behavior in pm.sample() when init=None

[ricardoV94]

When sample() is initiated with init=None, pymc3 does not try to call the general init_nuts() function, and instead silently defaults to the elementwise auto-assignment with assign_step_methods().

This behavior however is not explicitly mentioned in the documentation, where it is suggested that the init argument should always be a string with one of the listed options. Should we:

1. enforce this constraint, similarly to what is already done in init_nuts(), by raising TypeError and ValueError when an invalid init argument is passed?
2. document and allow this behavior, either by adding None as an explicit option in the documentation or adding a new string argument init="elementwise" that would work as None currently does.
3. do nothing...

I would be inclined to 1). If users want to avoid the default auto-assignment of the Nuts sampler, the documentation is quite clear that they should pass the desired step methods via the argument step.