prevent silent skip of validation #1594 (#1624)

* prevent silent skip of validation #1594

see https://github.com/open-mmlab/mmdetection/issues/1594

* correct for flake8

mmdet/apis/train.py

@@ -141,6 +141,11 @@ def build_optimizer(model, optimizer_cfg):
 
 
 def _dist_train(model, dataset, cfg, validate=False):
+    if validate:
+        raise NotImplementedError('Built-in validation is not implemented '
+                                  'yet in not-distributed training. Use '
+                                  'distributed training or test.py and '
+                                  '*eval.py scripts instead.')
     # prepare data loaders
     dataset = dataset if isinstance(dataset, (list, tuple)) else [dataset]
     data_loaders = [