[Serve] [Serve-autoscaler] Change target_num_ongoing_requests_per_replica to positive float

[zcin]

Signed-off-by: Cindy Zhang [email protected]

Why are these changes needed?

Allow for float values for target_num_ongoing_requests_per_replica.

Related issue number

"Closes #31209"

Checks

• [x] I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• [x] 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
  • [x] Unit tests
  • [ ] Release tests
  • [ ] This PR is not tested :(

[zcin]

Please lmk if anyone thinks there are other unit tests that I should update!