CSVBaseSampleLoader does not support delayed metadata

Since DelayedSample supports delayed metadata as well, I think it's a good idea that CSVBaseSampleLoader delays the metadata loading as well. This is really important as when we query the database, we may want to load the annotations in a delayed manner because they might not exist and annotaitons might not be used. see https://gitlab.idiap.ch/bob/bob.pipelines/-/blob/e2459dc5784045261ccc25df204a852bb527239e/bob/pipelines/datasets/sample_loaders.py#L60