Default value of `n_tpu_cores` in lightning_base.py
https://github.com/huggingface/transformers/blob/2804fff8393dbda5098b8c9f5e36235e89c50023/examples/lightning_base.py#L294

The default setting ``0`` raise Error
``pytorch_lightning.utilities.exceptions.MisconfigurationException: `tpu_cores` can only be 1, 8 or [<1-8>]``
It should be replaced by ``None``.