Update CHANGELOG.md

Author: ananthsub

CHANGELOG.md

@@ -8,6 +8,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 ## [1.3.0] - 2021-MM-DD
 
 ### Added
+- Added a `teardown` hook to `ClusterEnvironment` ([#6942](https://github.com/PyTorchLightning/pytorch-lightning/pull/6942))
+
 
 - Added utils for NaN/Inf detection for gradients and parameters ([#6834](https://github.com/PyTorchLightning/pytorch-lightning/pull/6834/))
 
@@ -189,6 +191,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 
 ### Fixed
+- Fixed incorrect removal of `WORLD_SIZE` environment variable in DDP training when launching with torch distributed/torchelastic ([#6942](https://github.com/PyTorchLightning/pytorch-lightning/pull/6942))
+
 
 - Set better defaults for `rank_zero_only.rank` when training is launched with SLURM and torchelastic:
     * Support SLURM and torchelastic global rank environment variables ([#5715](https://github.com/PyTorchLightning/pytorch-lightning/pull/5715))