a easy way to deploy

[CaoWGG]

a easy way to deploy and export onnx

[DingXiaoH]

I'm afraid this implementation is not consistent with the core idea of RepVGG. In this implementation, a RepVGG block always keeps three branches, and in the "deploy" mode it derives the equivalent kernel in advance for each forwarding. But in our design, the 1x1 and identity branches should be discarded right after training.