Add docstrings and improve comments (#11229)

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

docs/en/usage/callbacks.md

@@ -30,7 +30,7 @@ from ultralytics import YOLO
 
 
 def on_predict_batch_end(predictor):
-    # Retrieve the batch data
+    """Handle prediction batch end by combining results with corresponding frames; modifies predictor results."""
     _, image, _, _ = predictor.batch
 
     # Ensure that image is a list

docs/en/usage/engine.md

@@ -46,6 +46,7 @@ from ultralytics.models.yolo.detect import DetectionTrainer
 
 class CustomTrainer(DetectionTrainer):
     def get_model(self, cfg, weights):
+        """Loads a custom detection model given configuration and weight files."""
         ...
 
 
@@ -65,16 +66,19 @@ from ultralytics.nn.tasks import DetectionModel
 
 class MyCustomModel(DetectionModel):
     def init_criterion(self):
+        """Initializes the loss function and adds a callback for uploading the model to Google Drive every 10 epochs."""
         ...
 
 
 class CustomTrainer(DetectionTrainer):
     def get_model(self, cfg, weights):
+        """Returns a customized detection model instance configured with specified config and weights."""
         return MyCustomModel(...)
 
 
 # callback to upload model weights
 def log_model(trainer):
+    """Logs the path of the last model weight used by the trainer."""
     last_weight_path = trainer.last
     print(last_weight_path)