Use getter instead of python property for the dataloaders (#275)

* Use getter instead of python property for the dataloaders

* Fix lint

* Update trainer.py

pytorch_lightning/root_module/decorators.py

@@ -10,13 +10,18 @@ def data_loader(fn):
 
     attr_name = '_lazy_' + fn.__name__
 
-    @property
-    def _data_loader(self):
+    def _get_data_loader(self):
         try:
             value = getattr(self, attr_name)
         except AttributeError:
             try:
                 value = fn(self)  # Lazy evaluation, done only once.
+                if (
+                    value is not None and
+                    not isinstance(value, list) and
+                    fn.__name__ in['test_dataloader', 'val_dataloader']
+                ):
+                    value = [value]
             except AttributeError as e:
                 # Guard against AttributeError suppression. (Issue #142)
                 traceback.print_exc()
@@ -25,4 +30,4 @@ def data_loader(fn):
             setattr(self, attr_name, value)  # Memoize evaluation.
         return value
 
-    return _data_loader
+    return _get_data_loader