Fix annotation for api marker (#3099)

### Changes

- Rework api decorator to pass annotation of object
- Remove unused 'is_api' function

### Reason for changes

Broken annotation of object

docs/api/source/conf.py

@@ -75,7 +75,8 @@ def collect_api_entities() -> APIInfo:
         except Exception as e:
             skipped_modules[modname] = str(e)
 
-    from nncf.common.utils.api_marker import api
+    from nncf.common.utils.api_marker import API_MARKER_ATTR
+    from nncf.common.utils.api_marker import CANONICAL_ALIAS_ATTR
 
     canonical_imports_seen = set()
 
@@ -86,7 +87,7 @@ def collect_api_entities() -> APIInfo:
             if (
                 objects_module == modname
                 and (inspect.isclass(obj) or inspect.isfunction(obj))
-                and hasattr(obj, api.API_MARKER_ATTR)
+                and hasattr(obj, API_MARKER_ATTR)
             ):
                 marked_object_name = obj._nncf_api_marker
                 # Check the actual name of the originally marked object
@@ -95,8 +96,8 @@ def collect_api_entities() -> APIInfo:
                 if marked_object_name != obj.__name__:
                     continue
                 fqn = f"{modname}.{obj_name}"
-                if hasattr(obj, api.CANONICAL_ALIAS_ATTR):
-                    canonical_import_name = getattr(obj, api.CANONICAL_ALIAS_ATTR)
+                if hasattr(obj, CANONICAL_ALIAS_ATTR):
+                    canonical_import_name = getattr(obj, CANONICAL_ALIAS_ATTR)
                     if canonical_import_name in canonical_imports_seen:
                         assert False, f"Duplicate canonical_alias detected: {canonical_import_name}"
                     retval.fqn_vs_canonical_name[fqn] = canonical_import_name

nncf/common/utils/api_marker.py

@@ -8,24 +8,35 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from typing import Any
 
+from typing import Any, Callable, TypeVar, Union
 
-class api:
-    API_MARKER_ATTR = "_nncf_api_marker"
-    CANONICAL_ALIAS_ATTR = "_nncf_canonical_alias"
+TObj = TypeVar("TObj", bound=Union[Callable[..., Any], type])
 
-    def __init__(self, canonical_alias: str = None):
-        self._canonical_alias = canonical_alias
+API_MARKER_ATTR = "_nncf_api_marker"
+CANONICAL_ALIAS_ATTR = "_nncf_canonical_alias"
 
-    def __call__(self, obj: Any) -> Any:
-        # The value of the marker will be useful in determining
-        # whether we are handling a base class or a derived one.
-        setattr(obj, api.API_MARKER_ATTR, obj.__name__)
-        if self._canonical_alias is not None:
-            setattr(obj, api.CANONICAL_ALIAS_ATTR, self._canonical_alias)
-        return obj
 
+def api(canonical_alias: str = None) -> Callable[[TObj], TObj]:
+    """
+    Decorator function used to mark a object as an API.
+
+    Example:
+        @api(canonical_alias="alias")
+        class Class:
+            pass
+
+        @api(canonical_alias="alias")
+        def function():
+            pass
+
+    :param canonical_alias: The canonical alias for the API class.
+    """
+
+    def decorator(obj: TObj) -> TObj:
+        setattr(obj, API_MARKER_ATTR, obj.__name__)
+        if canonical_alias is not None:
+            setattr(obj, CANONICAL_ALIAS_ATTR, canonical_alias)
+        return obj
 
-def is_api(obj: Any) -> bool:
-    return hasattr(obj, api.API_MARKER_ATTR)
+    return decorator