Throw an exception if --force_exact_pieces specified on a non-composa…

…ble MWT dataset

stanza/models/mwt_expander.py

@@ -139,6 +139,9 @@ def train(args):
         logger.warning("Training data available, but dev data has no MWTs.  Only training a dict based MWT")
         args['dict_only'] = True
 
+    if args['force_exact_pieces'] and not mwts_composed_of_words(train_doc):
+        raise ValueError("Cannot train model with --force_exact_pieces, as the MWT in this dataset are not entirely composed of their subwords")
+
     if args['force_exact_pieces'] is None and mwts_composed_of_words(train_doc):
         # the force_exact_pieces mechanism trains a separate version of the MWT expander in the Trainer
         # (the training loop here does not need to change)
@@ -147,6 +150,8 @@ def train(args):
         # this behavior can be turned off at training time with --no_force_exact_pieces
         logger.info("Train MWTs entirely composed of their subwords.  Training the MWT to match that paradigm as closely as possible")
         args['force_exact_pieces'] = True
+
+    if args['force_exact_pieces']:
         logger.info("Reconverting to BinaryDataLoader")
         train_batch = BinaryDataLoader(train_doc, args['batch_size'], args, evaluation=False)
         vocab = train_batch.vocab