Classification : Implement Batch API for model evaluation

Current Behavior
For evaluation of fine-tuned models, we currently use the Responses API synchronously to generate predictions. These predictions are then compared against the true labels of the corresponding questions. However, this synchronous setup is difficult to manage and would require building a complex system to make the whole process asynchronous and more reliable.

Proposed Improvement
Instead of relying on the synchronous approach, we can leverage the OpenAI Batch API. By sending all evaluation queries together as a batch, we can retrieve the predictions from the batch output file and then compare them against the true labels. This would simplify the workflow and make the evaluation process more scalable and efficient.

document to refer : https://docs.google.com/document/d/1y2WLRUm2dZaQMetvb31XpDH5HqteRe4UNpziG8pq608/edit?tab=t.lbfxo8c09bqd#bookmark=id.1ib6z5f31fld

Review the current synchronous Responses API evaluation flow in the repository, then consult the linked design document for the proposed batch workflow. Done means evaluation queries are submitted through the Batch API, predictions are read from the batch output file, and those predictions are compared with the corresponding true labels.