Add backup for metrics.csv

[AntonioMirarchi]

We currently use CSVLogger from torch.lightning to write metrics to a CSV file in the log directory. However, when using load_model (i.e., loading from a checkpoint), or more in general to append data to an existing CSV, we overwrite the current metrics.csv, losing the original data.
This PR introduces a simple function that checks if the metrics file already exists and renames it using date and time information, preserving previous data.
Ideally, we would prefer to append to the existing file, but AFAIK this is not currently supported by CSVLogger.

[AntonioMirarchi]

It seems there is a problem with csvlogger.experiment.metrics_file_path when multiple GPUs are used for training. So I would just keep it simple and retrieve the path name as opj(log_dir, metrics.csv)