[PROBLEM] Retriving the results Batch API

[SarahHaKG]

[optional format]

Identify the file to be fixed
Not being able to retrive the results

Describe the problem
I am not able to retrive the results after finishing the job

, since the default batch_job.output_file_id is None. Here: examples/batch_processing.ipynb

Describe a solution
Give a way to set the batch_job.output_file_id

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