[MNT] Ensure base classes are abstract and adjust testing

Describe the issue

As discussed in https://github.com/aeon-toolkit/aeon/pull/2118#discussion_r1796538246 and Slack, our current testing framework checks that every estimator class has an implementation for _fit/_predict/_fit_predict using

>>> # Test that all anomaly detectors implement abstract predict.
>>> assert "_predict" in estimator_class.__dict__
E       AssertionError

This, however, just checks within the final class definition and ignores any potential implementations in the class hierarchy. Thus, in wrapper-land, we are required to overwrite these methods only to call the super-implementation, e.g.:

from aeon.anomaly_detection import PyODAdapter

class IsolationForest(PyODAdapter):
    def __init__(self, window_size, stride, **params):
        from pyod.models.iforest import IForest
        ...
        super().__init__(IForest(**params), window_size, stride)

    def _fit(self, X, y=None):
        super()._fit(X, y)

    def _predict(self, X):
        return super()._predict(X)

    def _fit_predict(self, X, y=None):
        return super()._fit_predict(X, y)

I would like to avoid the boilerplate code and reduce the wrapper to the class-definition, documentation, and constructor.

Suggest a potential alternative/fix

Python's abstract base classes already provide a way to enforce that abstract methods must be implemented. Otherwise, the instantiation of the class fails.

If we ensure that all our base classes inherit from abc.ABC and annotate their abstract methods with abc.abstractmethod, we can remove the testing for implementations of _fit/_predict/_fit_predict, and just test the instantiation of the estimator to not raise a TypeError: Can't instantiate abstract class XYZ with abstract method _xyz.

[x] ensure that all base classes inherit from abc.ABC
[ ] annotate all abstract methods with abc.abstractmethod (at least _predict, _fit, _fit_predict)
[ ] change testing framework (remove all checks for implementations of the abstract methods; add a test that instantiates an estimator and provides a proper error message)

Additional context

No response

aeon-toolkit / aeon