Automatically set sync_batchnorm for training_type_plugin

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -426,6 +426,11 @@ def resolve_training_type_plugin(self, training_type: TrainingTypePlugin) -> Tra
         if hasattr(training_type, 'num_nodes') and getattr(training_type, 'num_nodes') is None:
             training_type.num_nodes = self.num_nodes
 
+        # Automatically set sync_batchnorm.
+        # Useful for custom plugins.
+        if hasattr(training_type, 'sync_batchnorm'):
+            training_type.sync_batchnorm = self.sync_batchnorm
+
         return training_type
 
     def select_accelerator(self) -> Accelerator:

tests/plugins/test_custom_plugin.py

@@ -0,0 +1,22 @@
+from pytorch_lightning import Trainer
+from pytorch_lightning.plugins import DDPPlugin
+from tests.helpers import BoringModel
+
+
+class CustomParallelPlugin(DDPPlugin):
+    def __init__(self, **kwargs):
+        super().__init__(**kwargs)
+
+
+def test_sync_batchnorm_set(tmpdir):
+    model = BoringModel()
+    plugin = CustomParallelPlugin()
+    assert plugin.sync_batchnorm is False
+    trainer = Trainer(
+        max_epochs=1,
+        plugins=[plugin],
+        default_root_dir=tmpdir,
+        sync_batchnorm=True,
+    )
+    trainer.fit(model)
+    assert plugin.sync_batchnorm is True