fix mixup error when dataset has only 1 sample (#101)

* fix mixup error when dataset has only 1 sample

* refine log

ppdet/data/source/roidb_source.py

@@ -28,6 +28,9 @@ import copy
 import pickle as pkl
 from ..dataset import Dataset
 
+import logging
+logger = logging.getLogger(__name__)
+
 
 class RoiDbSource(Dataset):
     """ interface to load roidb data from files
@@ -134,6 +137,11 @@ class RoiDbSource(Dataset):
         if self._is_shuffle:
             random.shuffle(self._roidb)
 
+        if self._mixup_epoch > 0 and self._samples < 2:
+            logger.info("Disable mixup for dataset samples "
+                        "less than 2 samples")
+            self._mixup_epoch = -1
+
         if self._epoch < 0:
             self._epoch = 0
         else: