Mark accelerator connector as protected (#10032)

Summary:
### New commit log messages
  1f7bd6650 Mark accelerator connector as protected (#10032)

Reviewed By: yifuwang

Differential Revision: D31905981

fbshipit-source-id: a7f0f03033b02b603d28203ae2c8e8df4933fb23

d2go/runner/lightning_task.py

@@ -237,8 +237,8 @@ class DefaultTask(pl.LightningModule):
     def _reset_dataset_evaluators(self):
         """reset validation dataset evaluator to be run in EVAL_PERIOD steps"""
         assert (
-            not self.trainer.accelerator_connector.distributed_backend
-            or self.trainer.accelerator_connector.distributed_backend.lower()
+            not self.trainer._accelerator_connector.distributed_backend
+            or self.trainer._accelerator_connector.distributed_backend.lower()
             in ["ddp", "ddp_cpu"]
         ), ("Only DDP and DDP_CPU distributed backend are supported")