[Serve] Make max_batch_size and batch_wait_timeout_s reconfigurable

[shrekris-anyscale]

Why are these changes needed?

The @serve.batch decorator has two parameters: max_batch_size and batch_wait_timeout_s. These parameters can be set in the decorator. However, they cannot be reconfigured after the Serve application starts.

This change adds two setter methods: set_max_batch_size and set_batch_wait_timeout_s. Users can reconfigure their @serve.batch parameters using these methods:

@serve.batch(max_batch_size=1, batch_wait_timeout_s=0.1)
def batch_handler(self, request_list):
    ...

self.batch_handler.set_max_batch_size(5)
self.batch_handler.set_batch_wait_timeout_s(0.5)

Related issue number

Closes #36844

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
  • Unit tests
    • This change adds unit tests to test_batching.py.

[shrekris-anyscale]

@edoakes the code changes are ready for review. I'm planning on adding doc changes to this PR soon.

[shrekris-anyscale]

@edoakes I've added documentation. This PR is ready for review end-to-end.

[edoakes]

@shrekris-anyscale can you add a sentence to the @serve.batch docstring mentioning the setters?

[shrekris-anyscale]

@edoakes Thanks, I updated the docstring. Please take a look.