[tune] Fix PB2 scheduler error resulting from trying to sort by Trial objects

[ameroyer]

Why are these changes needed?

Very minor change solving a bug for the PB2 scheduler in tune: The dataframe storing the Trials should have its first column being of string type (for instance, here); However, in one case, it is stored as a Trial type instead, leading to a comparison error when the dataframe is sorted (which e.g. happens by default in the groupby operations)

Related issue number

Closes #37638.
(I believe this may also close #14187 although it's an older issue)

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 :(

[justinvyu]

Thank you for the fix!