Add ops.py comment for nm = number of masks (#13353)

Co-authored-by: UltralyticsAssistant <web@ultralytics.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/utils/ops.py

@@ -215,7 +215,7 @@ def non_max_suppression(
 
     bs = prediction.shape[0]  # batch size
     nc = nc or (prediction.shape[1] - 4)  # number of classes
-    nm = prediction.shape[1] - nc - 4
+    nm = prediction.shape[1] - nc - 4  # number of masks
     mi = 4 + nc  # mask start index
     xc = prediction[:, 4:mi].amax(1) > conf_thres  # candidates