PyTorch Hub cv2 .save() .show() bug fix (#2831)

* PyTorch Hub cv2 .save() .show() bug fix

cv2.rectangle() was failing on non-contiguous np array inputs. This checks for contiguous arrays and applies is necessary:
```python
imgs[i] = im if im.data.contiguous else np.ascontiguousarray(im)  # update
```

* Update plots.py

```python
assert im.data.contiguous, 'Image not contiguous. Apply np.ascontiguousarray(im) to plot_on_box() input image.'
```

* Update hubconf.py

Expand CI tests to OpenCV image.

hubconf.py

@@ -124,13 +124,15 @@ if __name__ == '__main__':
     # model = custom(path_or_model='path/to/model.pt')  # custom example
 
     # Verify inference
+    import cv2
     import numpy as np
     from PIL import Image
 
-    imgs = [Image.open('data/images/bus.jpg'),  # PIL
-            'data/images/zidane.jpg',  # filename
-            'https://github.com/ultralytics/yolov5/raw/master/data/images/bus.jpg',  # URI
-            np.zeros((640, 480, 3))]  # numpy
+    imgs = ['data/images/zidane.jpg',  # filename
+            'https://github.com/ultralytics/yolov5/releases/download/v1.0/zidane.jpg',  # URI
+            cv2.imread('data/images/bus.jpg')[:, :, ::-1],  # OpenCV
+            Image.open('data/images/bus.jpg'),  # PIL
+            np.zeros((320, 640, 3))]  # numpy
 
     results = model(imgs)  # batched inference
     results.print()