Migrating from `yapf` to `ruff format`

[hmellor]

Motivation.

We would like to transition vLLM from yapf to ruff format. This will give us:

• Increased line length
• Better formatting style which is more effectively enforced by tooling rather than by maintainers
• Fewer formatting tools fighting eachother

Proposed Change.

We plan to make this change gradually using the following process.

If we are converting directory x:

• In x, we add a local pyproject.toml which:
  • overrides ruff's line length to 88 (it's own default)
  • removes the deprecated type ignores
  • enables isort in ruff
  • enables formatting of code in docstrings (good for the API docs)
• We add x to the list of files to run ruff-format on in .pre-commit-config.yaml
• We add x to the list of ignores in the yapf and isort config in the root pyproject.toml

Here is the list of PRs used to make the transition:

• Convert .buildkite to ruff format #17656
• Convert benchmarks to ruff format #18068
• Convert examples to ruff-format #18400
• Convert tests to ruff-format #21129
• Convert formatting to use ruff instead of yapf + isort #26247

[punitvara]

Is there scope remaining to convert yapf to ruff format ? @hmellor

[hmellor]

From internal discussion with @zhuohan123 the plan is as follows:

1. Fully delete the last of V0
2. Immediately make a PR which fully converts the whole remaining codebase in one go