[ENH] clone method to handle nested dict (#362)

This adds the ability to the `clone` methods to also clone nested
`dict`-s.

Closes #280, see
#280 (comment)

skbase/base/_base.py

@@ -1384,7 +1384,8 @@ def _clone(estimator, *, safe=True):
     found in :ref:`randomness`.
     """
     estimator_type = type(estimator)
-    # XXX: not handling dictionaries
+    if estimator_type is dict:
+        return {k: _clone(v, safe=safe) for k, v in estimator.items()}
     if estimator_type in (list, tuple, set, frozenset):
         return estimator_type([_clone(e, safe=safe) for e in estimator])
     elif not hasattr(estimator, "get_params") or isinstance(estimator, type):