Correct YOLO yaml name, input variable in _Conv2d, and package import in boxes

[anthonyweidai]

1. Modify all names of yamls "Base-YoloV7.yaml" as "Base-YOLOv7.yaml".
2. Variable 'x' is not defined. Change it to "input".
3. Remove repeated and unused imported packages. Rearrange them for elegance.