Make output_dir Optional in TrainingArguments #27866

[sambhavnoobcoder]

Description

This PR addresses an issue where specifying an output_dir was mandatory in TrainingArguments. This behavior could be confusing or cumbersome in scenarios where users do not necessarily need a dedicated output directory or prefer to let the library handle default paths.

By making output_dir optional and defaulting to a temporary directory (named "tmp_trainer") when not specified, we simplify the API usage. If a user later needs to track outputs in a dedicated directory, they can still provide a valid path.

Fixes : #27866

Proposed Solution

• Allow output_dir to be set to None.
• Default to a built-in location (e.g., "tmp_trainer") if None is provided.

Implementation

The changes introduce a check to automatically assign a default directory if none is specified. This preserves backward compatibility for users already specifying output_dir while improving the experience for those who prefer a simpler workflow.

Testing

• Added a test covering the scenario when output_dir is not provided.
• Verified existing tests using a custom output_dir are unaffected.
• Extended tests to ensure the directory is only created when needed (depending on the save strategy).

Screenshots

Below is a screenshot of all testcases passing :

cc : @ArthurZucker @Rocketknight1

[Rocketknight1]

cc @SunMarc @muellerzr for Trainer!