Training breaks if filelist database has only 'norm' group
If a filelist database has only training date inside 'norm' folder (of a given protocol) but does has neither 'dev' or 'eval', the
verify.py script when used for training only, e.g., by running train_gmm.py, train_isv.py, or other derivatives, will break in
uses_dense_probe_file() function here:
uses_dense_probe_file() assumes that there must be either 'dev' or 'eval' sets. But it may not be the case if we just want to train a UBM on the whole database (so we put all data inside 'norm').