Custom Plugin is_distributed (#6537)

* return from plugin

* dont return for tpu

(cherry picked from commit 6a14146)

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -266,6 +266,10 @@ def use_deepspeed(self) -> bool:
 
     @property
     def is_distributed(self) -> bool:
+        # Used for custom plugins.
+        # Custom plugins should implement is_distributed property.
+        if hasattr(self.training_type_plugin, 'is_distributed') and not self.on_tpu:
+            return self.training_type_plugin.is_distributed
         is_distributed = self.use_ddp or self.use_ddp2 or self.use_horovod
         if self.on_tpu:
             is_distributed |= self.training_type_plugin.is_distributed