[bugfix] Check LightningOptimizer doesn't delete optimizer hooks (#6305)

* update

* resolve bug

Author: tchaton

CHANGELOG.md

@@ -63,6 +63,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Ensure we check deepspeed/sharded in multinode DDP ([#6297](https://github.com/PyTorchLightning/pytorch-lightning/pull/6297)
 
 
+- Check `LightningOptimizer` doesn't delete optimizer hooks ([#6305](https://github.com/PyTorchLightning/pytorch-lightning/pull/6305)
+
+
 ## [1.2.2] - 2021-03-02
 
 ### Added