Small fixes for reading the input files correctly and to the OpenAIFi…

[robtinn]

Summary

The first of a few PRs I will do to update and extend the content in this cookbook example.

This PR will make some small fixes:

• attribute style access for self.file_object
• ensuring the training file is read as binary to avoid errors
• updating the wait_for_finetuning method so it uses retrieve() instead of refresh() and exits on success, failure or job cancellation.

Motivation

Some of these issues have been picked up in the GitHub issues and it will make the cookbook easier to run without users having to make several fixes.