Better diagnostics: Regression error plots

[ant0nsc]

Regression models should at rank zero write this a prediction/target plot like this in the old code:

    if self._should_save_regression_error_plot(self.current_epoch):
        error_plot_name = f"error_plot_{self.train_val_params.epoch}"
        path = str(self.config.outputs_folder / f"{error_plot_name}.png")
        plot_variation_error_prediction(epoch_metrics.get_labels(), epoch_metrics.get_predictions(), path)
        logger = self.config.azure_loggers_train if is_training else self.config.azure_loggers_val
        logger.log_image(error_plot_name, path)

AB#3914