Allow dataloader_idx_ in log names when add_dataloader_idx=False (#20987)

(cherry picked from commit 37f559e)

Author: rittik9

src/lightning/pytorch/CHANGELOG.md

@@ -10,7 +10,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Changed
 
--
+- Allow `dataloader_idx_` in log names when `add_dataloader_idx=False` ([#20987](https://github.com/Lightning-AI/pytorch-lightning/pull/20987))
 
 
 ### Fixed

src/lightning/pytorch/core/module.py

@@ -466,10 +466,10 @@ def log(
         )
 
         # make sure user doesn't introduce logic for multi-dataloaders
-        if "/dataloader_idx_" in name:
+        if add_dataloader_idx and "/dataloader_idx_" in name:
             raise MisconfigurationException(
                 f"You called `self.log` with the key `{name}`"
-                " but it should not contain information about `dataloader_idx`"
+                " but it should not contain information about `dataloader_idx` when `add_dataloader_idx=True`"
             )
 
         value = apply_to_collection(value, (Tensor, numbers.Number), self.__to_tensor, name)