[doc] Fix doc building

doc/extras.txt

@@ -2,6 +2,7 @@
 # Project: extras
 # Version: stable
 # The remainder of this file is compressed using zlib.
+torch.optim.lr_scheduler.LRScheduler py:class 1 https://pytorch.org/docs/stable/_modules/torch/optim/lr_scheduler.html -
 torchvision.transforms py:module 1 https://pytorch.org/vision/stable/transforms.html -
 optimizer_step py:method 1 api/lightning.pytorch.core.LightningModule.html#$ -
 json.encoder.JSONEncoder py:class 1 https://docs.python.org/3/library/json.html#json.JSONEncoder -