Add val.py no label warning (#8782)

Help resolve confusion around zero-metrics val.py results when no labels are found in https://github.com/ultralytics/yolov5/issues/8753

Add val.py no label warning (#8782)
e309a855 · Glenn Jocher · GitHub · 567397d6 · e309a855
--- a/val.py
+++ b/val.py
@@ -275,6 +275,8 @@ def run(
    # Print results
    pf = '%20s' + '%11i' * 2 + '%11.3g' * 4  # print format
    LOGGER.info(pf % ('all', seen, nt.sum(), mp, mr, map50, map))
+    if nt.sum() == 0:
+        LOGGER.warning(emojis(f'WARNING: no labels found in {task} set, can not compute metrics without labels ⚠️'))

    # Print results per class
    if (verbose or (nc < 50 and not training)) and nc > 1 and len(stats):