text: Title: Drop `cv2` from dev dependencies
  Body: ## 🚀 Feature
  The cv2 package is just used in one place at tests. So, we can figure out how to test the case without it
  https://github.com/kornia/kornia/blob/216aa9d2f10f9300bffe952b1583f03b7f833be5/test/io/test_io_image.py#L4
  
  ## Motivation
  reduce the number of dependencies at kornia 