add changelog

Author: awaelchli

CHANGELOG.md

@@ -131,6 +131,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed `Trainer` not resetting `lightning_optimizers` when calling `Trainer.fit()` multiple times ([#6372](https://github.com/PyTorchLightning/pytorch-lightning/pull/6372))
 
 
+- Fixed an issue where the tuner would not tune the learning rate if also tuning the batch size ([#4688](https://github.com/PyTorchLightning/pytorch-lightning/pull/4688))
+
+
 ## [1.2.2] - 2021-03-02
 
 ### Added