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Introduce formatter

See original GitHub issue

Motivation

If we introduce a formatter to Optuna and just use it, we do not need to think about formatting.

Description

There are famous formatters in python:

autopep8 (current)
Black
- example of applying black #957
yapf

c.f. https://blog.frank-mich.com/python-code-formatters-comparison-black-autopep8-and-yapf/

We need to discuss what formatter is best.

Differences of above formatters

black

Use " instead of '.
Add trailing commas to expressions that are split by comma where each element is on its own line.
Break a line before a binary operator when splitting a block of code over multiple lines.

black vs yapf

c.f. https://news.ycombinator.com/item?id=17155048

YAPF would at times not produce deterministic formatting (formatting the same file the second time with no changes in between would create a different formatting); Black treats this as a bug;

YAPF would not format all files that use the latest Python 3.6 features (we have a lot of f-strings, there’s cases of async generators, complex unpacking in collections and function calls, and so on); Black solves that;

YAPF is based on a sophisticated algorithm that unwinds the line and applies “penalty points” for things that the user configured they don’t like to see. With a bit of dynamic programming magic it arrives at a formatting with the minimal penalty value. This works fine most of the time. When it doesn’t, and surprised people ask you to explain, you don’t really know why. You might be able to suggest changing the penalty point value of a particular decision from, say, 47 to 48. It might help with this particular situation… but break five others in different places of the codebase.