add changelog

Author: tchaton

CHANGELOG.md

@@ -5,7 +5,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 
-## [UnReleased] - 2021-MM-DD
+## [1.2.3] - 2021-MM-DD
+
+### Fixed
+
+
+- Fixed DP reduction with collection ([#6324](https://github.com/PyTorchLightning/pytorch-lightning/pull/6324))
+
+
+## [1.2.2] - 2021-03-03
 
 ### Added
 
@@ -101,6 +109,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed error thrown when using valid distributed mode in multi node ([#6297](https://github.com/PyTorchLightning/pytorch-lightning/pull/6297)
 
 
+- Added `checkpoint` parameter to callback's `on_save_checkpoint` hook ([#6072](https://github.com/PyTorchLightning/pytorch-lightning/pull/6072))
+
 ## [1.2.1] - 2021-02-23
 
 ### Fixed