Allow find_unused_parameters to be set in config (#2225)

* Allow find_unused_parameters to be set in config

* Fix typo

mmdet/apis/train.py

@@ -129,10 +129,14 @@ def _dist_train(model,
             seed=cfg.seed) for ds in dataset
     ]
     # put model on gpus
+    find_unused_parameters = cfg.get('find_unused_parameters', False)
+    # Sets the `find_unused_parameters` parameter in
+    # torch.nn.parallel.DistributedDataParallel
     model = MMDistributedDataParallel(
         model.cuda(),
         device_ids=[torch.cuda.current_device()],
-        broadcast_buffers=False)
+        broadcast_buffers=False,
+        find_unused_parameters=find_unused_parameters)
 
     # build runner
     optimizer = build_optimizer(model, cfg.optimizer)