Unified model loading with backwards compatibility (#132)

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

docs/quickstart.md

@@ -42,9 +42,9 @@ Ultralytics YOLO comes with pythonic Model and Trainer interface.
     import ultralytics
     from ultralytics import YOLO
 
-    model = YOLO("s-seg.yaml") # automatically detects task type
-    model = YOLO("s-seg.pt") # load checkpoint
-    model.train(data="coco128-segments", epochs=1, lr0=0.01, ...)
-    model.train(data="coco128-segments", epochs=1, lr0=0.01, device="0,1,2,3") # DDP mode
+    model = YOLO("yolov8n-seg.yaml") # automatically detects task type
+    model = YOLO("yolov8n.pt") # load checkpoint
+    model.train(data="coco128-seg.yaml", epochs=1, lr0=0.01, ...)
+    model.train(data="coco128-seg.yaml", epochs=1, lr0=0.01, device="0,1,2,3") # DDP mode
     ```
 [API Guide](sdk.md){ .md-button .md-button--primary}