[WIP] Add unit test for tune api

[helenxie-bit]

What this PR does / why we need it:
This PR adds unit test for tune API.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Checklist:

• For a detailed explanation of this API, please refer to the proposal: [GSoC] Create LLM Hyperparameters Optimization API Proposal #2333

[google-oss-prow]

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[helenxie-bit]

/area gsoc

[helenxie-bit]

Ref: #2339