Implement all missing docstrings (#5298)

Co-authored-by: snyk-bot <snyk-bot@snyk.io>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

ultralytics/engine/__init__.py

@@ -0,0 +1 @@
+# Ultralytics YOLO 🚀, AGPL-3.0 license

ultralytics/engine/exporter.py

@@ -140,7 +140,7 @@ class Exporter:
         Args:
             cfg (str, optional): Path to a configuration file. Defaults to DEFAULT_CFG.
             overrides (dict, optional): Configuration overrides. Defaults to None.
-            _callbacks (list, optional): List of callback functions. Defaults to None.
+            _callbacks (dict, optional): Dictionary of callback functions. Defaults to None.
         """
         self.args = get_cfg(cfg, overrides)
         if self.args.format.lower() in ('coreml', 'mlmodel'):  # fix attempt for protobuf<3.20.x errors