Add on_fit_epoch_end callback (#5232)

* Add `on_fit_epoch_end` callback

* Add results to train

* Update __init__.py

train.py

@@ -423,8 +423,10 @@ def train(hyp,  # path/to/hyp.yaml or hyp dictionary
                                             plots=True,
                                             callbacks=callbacks,
                                             compute_loss=compute_loss)  # val best model with plots
+                    if is_coco:
+                        callbacks.run('on_fit_epoch_end', list(mloss) + list(results) + lr, epoch, best_fitness, fi)
 
-        callbacks.run('on_train_end', last, best, plots, epoch)
+        callbacks.run('on_train_end', last, best, plots, epoch, results)
         LOGGER.info(f"Results saved to {colorstr('bold', save_dir)}")
 
     torch.cuda.empty_cache()