[MaskRCNN] Allow cfg.DATA.TRAIN to be a string (#1276)

examples/FasterRCNN/config.py

@@ -221,6 +221,8 @@ def finalize_configs(is_training):
     _C.freeze(False)  # populate new keys now
     if isinstance(_C.DATA.VAL, six.string_types):  # support single string (the typical case) as well
         _C.DATA.VAL = (_C.DATA.VAL, )
+    if isinstance(_C.DATA.TRAIN, six.string_types):  # support single string
+        _C.DATA.TRAIN = (_C.DATA.TRAIN, )
 
     assert _C.BACKBONE.NORM in ['FreezeBN', 'SyncBN', 'GN', 'None'], _C.BACKBONE.NORM
     if _C.BACKBONE.NORM != 'FreezeBN':

examples/FasterRCNN/data.py

@@ -332,7 +332,6 @@ def get_train_dataflow():
 
     If MODE_MASK, gt_masks: (N, h, w)
     """
-
     roidbs = list(itertools.chain.from_iterable(DatasetRegistry.get(x).training_roidbs() for x in cfg.DATA.TRAIN))
     print_class_histogram(roidbs)