Fix MetricGroup requiring (y_pred, y) keys on mapping outputs - #3808

Changed pages include:

• index.html - view
• _modules/index.html - view
• defaults.html - view
• distributed.html - view
• engine.html - view
• exceptions.html - view
• genindex.html - view
• handlers.html - view
• metrics.html - view
• py-modindex.html - view

There are more changes not included in the list