validate api transfer to hook

tools/train.py

@@ -134,12 +134,6 @@ def main():
     runner = Runner(model, batch_processor, cfg.optimizer, cfg.work_dir,
                     cfg.log_level)
 
-    if args.validate:
-        val_dataset = obj_from_dict(cfg.data.test, datasets)
-        data_loaders.append(
-            build_dataloader(val_dataset, cfg.data.imgs_per_gpu,
-                             cfg.data.workers_per_gpu, cfg.gpus, dist))
-
     # register hooks
     optimizer_config = DistOptimizerHook(
         **cfg.optimizer_config) if dist else cfg.optimizer_config
@@ -148,6 +142,7 @@ def main():
     if dist:
         runner.register_hook(DistSamplerSeedHook())
         # register eval hooks
+        if args.validate:
             if isinstance(model.module, RPN):
                 runner.register_hook(CocoDistEvalRecallHook(cfg.data.val))
             elif cfg.data.val.type == 'CocoDataset':