[Enhancement] Support custom runner (#1923)

* support custom runner

* fix

tools/train.py

@@ -6,6 +6,7 @@ import os.path as osp
 
 from mmengine.config import Config, DictAction
 from mmengine.logging import print_log
+from mmengine.registry import RUNNERS
 from mmengine.runner import Runner
 
 from mmseg.utils import register_all_modules
@@ -97,7 +98,13 @@ def main():
         cfg.load_from = args.resume
 
     # build the runner from config
-    runner = Runner.from_cfg(cfg)
+    if 'runner_type' not in cfg:
+        # build the default runner
+        runner = Runner.from_cfg(cfg)
+    else:
+        # build customized runner from the registry
+        # if 'runner_type' is set in the cfg
+        runner = RUNNERS.build(cfg)
 
     # start training
     runner.train()