Remove deprecated `distributed_backend` from `Trainer` (#10017)

Summary:
### New commit log messages
  0aa220b46 Remove deprecated `distributed_backend` from `Trainer` (#10017)

Reviewed By: kandluis

Differential Revision: D31788128

fbshipit-source-id: 4d9394e119b3122014fc9681a5c56aac8df49141

d2go/runner/lightning_task.py

@@ -237,15 +237,10 @@ class DefaultTask(pl.LightningModule):
     def _reset_dataset_evaluators(self):
         """reset validation dataset evaluator to be run in EVAL_PERIOD steps"""
         assert (
-            not self.trainer.distributed_backend
-            or self.trainer.distributed_backend.lower()
-            in [
-                "ddp",
-                "ddp_cpu",
-            ]
-        ), (
-            "Only DDP and DDP_CPU distributed backend are supported"
-        )
+            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")
 
         def _get_inference_dir_name(
             base_dir, inference_type, dataset_name, model_tag: ModelTag