Error on zero length dataloaders (#1280)

* error_on_zero_length

* update CHANGELOG.md

* added test

* Update pytorch_lightning/trainer/data_loading.py

Co-authored-by: Nicki Skafte <nugginea@gmail.com>
Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

tests/base/__init__.py

@@ -25,7 +25,8 @@ from tests.base.mixins import (
     LightTestOptimizerWithSchedulingMixin,
     LightTestMultipleOptimizersWithSchedulingMixin,
     LightTestOptimizersWithMixedSchedulingMixin,
-    LightTestReduceLROnPlateauMixin
+    LightTestReduceLROnPlateauMixin,
+    LightZeroLenDataloader
 )
 
 

tests/base/mixins.py

@@ -252,6 +252,16 @@ class LightInfTestDataloader:
         return CustomInfDataloader(self._dataloader(train=False))
 
 
+class LightZeroLenDataloader:
+    """ Simple dataloader that has zero length. """
+
+    def train_dataloader(self):
+        dataloader = self._dataloader(train=True)
+        dataloader.dataset.data = dataloader.dataset.data[:0]
+        dataloader.dataset.targets = dataloader.dataset.targets[:0]
+        return dataloader
+
+
 class LightEmptyTestStep:
     """Empty test step."""