Cleanup yolo and remove stateless classes (tinygrad#604)

* Add AvgPool2d as a layer

* Clean up a bit

* Remove stateless layers in yolo_nn

* More cleanup

* Save label for test

* Add test for YOLO

* Test without cv2

* Don't fail if cv2 not installed

* Better import

* Fix image read

* Use opencv :)

* Don't download the file

* Fix errors

* Use same version

* Set higher confidence

* Why is the confidence so low?

* Start over

* Remove stateless layers

* Remove extra lines

* Revert changes

* Save a few more lines