[Tune] `bohb_example` patch fix

[justinvyu]

Why are these changes needed?

The bohb_example is super flaky after https://github.com/ray-project/ray/pull/35338 modified the example to run more trials. This is just a patch fix to deflake the example. There is still some underlying root cause to investigate. The other BOHB example (the Jupyter notebook) is also flaky, but for a different reason. See https://github.com/ray-project/ray/issues/35428.

In general, I unsure about the current hyperband implementation. See comments below for parts that I don't really understand.

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 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
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