[Serve] Allow methods to pass type @serve.batch type hint

[shrekris-anyscale]

Why are these changes needed?

Without this change, mypy raises a type warning when decorating methods with @serve.batch:

With this change, mypy no longer raises a type warning for methods:

We should be aware that functions with an extra parameter before the input list will now pass the type check. This allows users to define a method outside a class and then assign it to a class:

Related issue number

N/A

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • This change relies on existing tests.

[edoakes]

thx!