Decouple utilities from `LightningLoggerBase` (#11484)

Summary:
### New commit log messages
- [115a5d08e Decouple utilities from `LightningLoggerBase` (#11484)](https://github.com/PyTorchLightning/pytorch-lightning/pull/11484)

Reviewed By: tangbinh, wat3rBro

Differential Revision: D33960185

fbshipit-source-id: 6be72ad49f8433be6f238b36aa82d3f1b655e6f0

d2go/runner/lightning_task.py

@@ -40,6 +40,7 @@ from detectron2.solver import (
     build_optimizer as d2_build_optimizer,
 )
 from pytorch_lightning.utilities import rank_zero_only, rank_zero_info
+from pytorch_lightning.utilities.logger import _flatten_dict
 
 _STATE_DICT_KEY = "state_dict"
 _OLD_STATE_DICT_KEY = "model"
@@ -222,7 +223,7 @@ class DefaultTask(pl.LightningModule):
             nested_res[tag.lower()] = res
 
         self.eval_res = nested_res
-        flattened = pl.loggers.LightningLoggerBase._flatten_dict(nested_res)
+        flattened = _flatten_dict(nested_res)
 
         if self.trainer.global_rank:
             assert (