Make core project installable via pip

[lukazso]

I made the yolov6 directory with its contents installable via pip by adding setup.py and putting the directories in the package structure (simply added empty __init__.pys).

You can now install everything inside the yolov6 directory via pip by either

1. Cloning the project and then running pip install . inside the project root directory, or
2. Directly install it from github by running pip install git+https://github.com/meituan/YOLOv6.git (if it is on the main branch)

[Chilicyy]

Thanks for your contribution! We will verify and test the code as soon as possible.

[kadirnar]

Hello @Chilicyy ,

I need the pip version for my projects. When will you accept this PR?