Document that train and eval files are not always required
model_dir: toy-ende
data:
+ # (required for train_and_eval and train run types).
train_features_file: data/toy-ende/src-train.txt
train_labels_file: data/toy-ende/tgt-train.txt
+
+ # (required for train_end_eval and eval run types).
eval_features_file: data/toy-ende/src-val.txt
eval_labels_file: data/toy-ende/tgt-val.txt
