[air] Render trainer docstring signatures

[richardliaw]

Why are these changes needed?

Because we override __new__, we can't render signatures correctly, losing valuable type hints. This is a one-off fix to improve this by rendering the init method manually, but it is not a great fix. Ideally we get rid of our usage of __new__ and render things correctly.

Related issue number

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 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
  • Unit tests
  • Release tests
  • This PR is not tested :(

[pcmoritz]

Yeah it is inconsistent with the rest now (__init__ vs showing types in the class constructor), we should try to fix it properly. Fine as a workaround.