[BUG] Fix Sklearn Models detection by safely importing openml-sklearn (#1556)

#### Metadata

* Reference Issue: Fixes #1542
 
#### Details 
Fixed sklearn models detection by safely importing openml-sklearn at `openml/runs/__init__.py`

Author: EmanAbdelhaleem

openml/extensions/functions.py

@@ -1,6 +1,7 @@
 # License: BSD 3-Clause
 from __future__ import annotations
 
+import importlib.util
 from typing import TYPE_CHECKING, Any
 
 # Need to implement the following by its full path because otherwise it won't be possible to
@@ -16,8 +17,9 @@
 SKLEARN_HINT = (
     "But it looks related to scikit-learn. "
     "Please install the OpenML scikit-learn extension (openml-sklearn) and try again. "
+    "You can use `pip install openml-sklearn` for installation."
     "For more information, see "
-    "https://github.com/openml/openml-sklearn?tab=readme-ov-file#installation"
+    "https://docs.openml.org/python/extensions/"
 )
 
 
@@ -58,6 +60,10 @@ def get_extension_by_flow(
     -------
     Extension or None
     """
+    # import openml_sklearn to register SklearnExtension
+    if importlib.util.find_spec("openml_sklearn"):
+        import openml_sklearn  # noqa: F401
+
     candidates = []
     for extension_class in openml.extensions.extensions:
         if extension_class.can_handle_flow(flow):
@@ -103,6 +109,10 @@ def get_extension_by_model(
     -------
     Extension or None
     """
+    # import openml_sklearn to register SklearnExtension
+    if importlib.util.find_spec("openml_sklearn"):
+        import openml_sklearn  # noqa: F401
+
     candidates = []
     for extension_class in openml.extensions.extensions:
         if extension_class.can_handle_model(model):