Add `--conf-thres` >> 0.001 warning (#5567)

Partially addresses invalid mAPs at higher confidence threshold issue https://github.com/ultralytics/yolov5/issues/1466.

val.py

@@ -330,6 +330,8 @@ def main(opt):
     check_requirements(requirements=ROOT / 'requirements.txt', exclude=('tensorboard', 'thop'))
 
     if opt.task in ('train', 'val', 'test'):  # run normally
+        if opt.conf_thres > 0.001:  # https://github.com/ultralytics/yolov5/issues/1466
+            LOGGER.info(f'WARNING: confidence threshold {opt.conf_thres} >> 0.001 will produce invalid mAP values.')
         run(**vars(opt))
 
     elif opt.task == 'speed':  # speed benchmarks