fix: SDK list_checkpoints not defaulting to searcher metric sort

[azhou-determined]

Description

calling list_checkpoints without sort and order parameters is supposed to default to the experiment's searcher config, but instead falls through to sorting by ID on the backend.

Test Plan

Ensure that calling list_checkpoints without any arguments returns a list of checkpoints sorted by the experiment's searcher metric and smaller_is_better

client = experimental.Determined(master=DET_MASTER, user="determined", password="")
    ckpts = client.get_experiment(1).list_checkpoints()
    print(ckpts)

Commentary (optional)

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

Ticket

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[wes-turner]

Sigh. Probably was safer to do it this way in the first place. I'm often finding myself curious about the defaults the backend uses.

LGTM

I don't think this needs a regression test, but mentioning it here in case you think different and hadn't considered it.