We need to have a design decision on conflicting elements of our current interface specification.

These are:

1. __init__ should contain no logic for input checking, and optimally no other logic. Such logic should be placed at the start of _fit
2. dynamic tags need to be set in the __init__, since tags are read at the start of the fit call which precedes the _fit call
3. we have the fit_is_empty tag, which can mean that _fit is never called for a given estimator, as well as estimators/object types that have no fit (such as distances)

Here, 1. conflucts with 2. and with 3., but 2. and 3. do not conflict with each other (I think).

I feel we need to think about potential ways to resolve this situation.

Related: #1798