[tune] Increase the minimum number of allowed pending trials for faster auto-scaleup

[justinvyu]

Why are these changes needed?

This PR bumps up the minimum number of allowed pending trials from 16 to 200. This increases the speed of autoscaling for a Tune job that starts with a small cluster.

Ray Core benchmarks test submitting up to 600+ placement groups at a time. Having too many pending placement groups affects actor scheduling latency, so allowing up to 200 pending PGs is a middle ground.

Related issue number

Mitigation for #43451

This does not fully fix the issue, since it may be better to adjust this value dynamically in response to cluster scale-ups. Currently, the value gets fixed to max(16, int(initial_cluster_cpus * 1.1), where initial_cluster_cpus is just the number of CPUs at the beginning of the job.

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
  • Release tests
  • This PR is not tested :(

[woshiyyya]

LGTM