dont check for tracking uri when running mlflow offline

[cathalobrien]

The old behavior when running offline with the default tracking uri ('???') would cause a crash. So now tracking uri isnt checked when running offline

    offline = config.diagnostics.log.mlflow.offline
    if not offline:
        tracking_uri = config.diagnostics.log.mlflow.tracking_uri
        LOGGER.info("AnemoiMLFlow logging to %s", tracking_uri)
    else:
        tracking_uri = None

My understanding is there is no reason to check tracking uri when running offline because tracking_uri is provided later if the run is synced to mlflow.
One use case for running offline without a tracking uri would be for external vendors benchmarking anemoi. They could use the mlflow offline logging to analyse GPU utilization etc.