Update Detections() self.n comment (ultralytics#2620)

```python
        self.n = len(self.pred)  # number of images (batch size)
```

models/common.py

@@ -247,7 +247,7 @@ def __init__(self, imgs, pred, files, times=None, names=None, shape=None):
         self.xywh = [xyxy2xywh(x) for x in pred]  # xywh pixels
         self.xyxyn = [x / g for x, g in zip(self.xyxy, gn)]  # xyxy normalized
         self.xywhn = [x / g for x, g in zip(self.xywh, gn)]  # xywh normalized
-        self.n = len(self.pred)
+        self.n = len(self.pred)  # number of images (batch size)
         self.t = tuple((times[i + 1] - times[i]) * 1000 / self.n for i in range(3))  # timestamps (ms)
         self.s = shape  # inference BCHW shape