Remove scikit-learn extension as submodule, publish independently instead (openml#1424)

* Delete the extension

* Remove scikit-learn extension submodule

It will now be hosted in a separate repository

* Do not load sklearn extension by default

* Disable scikit-learn tests

* Tests fail successfully

* Add openml-sklearn as dependency of sklearn tests

* Make use of openml_sklearn extension

* packaging is only used in test submodules

Author: PGijsbers

openml/__init__.py

@@ -121,7 +121,3 @@ def populate_cache(
     "_api_calls",
     "__version__",
 ]
-
-# Load the scikit-learn extension by default
-# TODO(eddiebergman): Not sure why this is at the bottom of the file
-import openml.extensions.sklearn  # noqa: E402, F401

openml/extensions/functions.py

@@ -13,6 +13,13 @@
 
     from . import Extension
 
+SKLEARN_HINT = (
+    "But it looks related to scikit-learn. "
+    "Please install the OpenML scikit-learn extension (openml-sklearn) and try again. "
+    "For more information, see "
+    "https://github.com/openml/openml-sklearn?tab=readme-ov-file#installation"
+)
+
 
 def register_extension(extension: type[Extension]) -> None:
     """Register an extension.
@@ -57,7 +64,13 @@ def get_extension_by_flow(
             candidates.append(extension_class())
     if len(candidates) == 0:
         if raise_if_no_extension:
-            raise ValueError(f"No extension registered which can handle flow: {flow}")
+            install_instruction = ""
+            if flow.name.startswith("sklearn"):
+                install_instruction = SKLEARN_HINT
+            raise ValueError(
+                f"No extension registered which can handle flow: {flow.flow_id} ({flow.name}). "
+                f"{install_instruction}"
+            )
 
         return None
 
@@ -96,7 +109,12 @@ def get_extension_by_model(
             candidates.append(extension_class())
     if len(candidates) == 0:
         if raise_if_no_extension:
-            raise ValueError(f"No extension registered which can handle model: {model}")
+            install_instruction = ""
+            if type(model).__module__.startswith("sklearn"):
+                install_instruction = SKLEARN_HINT
+            raise ValueError(
+                f"No extension registered which can handle model: {model}. {install_instruction}"
+            )
 
         return None