placing the run dir only in the output_dir
Hi
In training_args.py, there is a code which creates a run dir, then anyplace user runs the code there it would create a run dir, could you create it only in the output_dir? thanks 


```
def default_logdir() -> str:
    """
    Same default as PyTorch
    """
    import socket
    from datetime import datetime

    current_time = datetime.now().strftime("%b%d_%H-%M-%S")
    return os.path.join("runs", current_time + "_" + socket.gethostname())

```


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