Created by: reyoung

Use a python package pre-commit to manage git pre-commit hooks when developing Paddle.

Git pre-commit hook is set by each developer locally. It is hard to force all developer set same pre-commit hooks, but there are some basic things should be check before commit, such as python style, file should use \n as line end, etc.

This PR is added to solve this problem, by using a python package pre-commit. Each developer should do two things to enable this feature.

• Install pre-commit package. pip install pre-commit
• Enable pre-commit package to manage Paddle git pre-commit in local directory, by run pre-commit install in Paddle checkout directory.

Currently, some hooks are added, and all files are scan & fixed by this PR.

By the way, there is another thing I found about yapf, the python formatter used by Paddle. Different version of yapf could generate different result, with the same input python file. So, we must use the same yapf when developing Paddle. pre-commit also can help us to archive that.

pre-commit is used only by Paddle developer, we should add some other tools (or just use pre-commit package, I'm not sure) to complete the CI tests about these checks.