add changelog

Author: awaelchli

CHANGELOG.md

@@ -90,6 +90,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed recursive passing of `wrong_type` keyword argument in `pytorch_lightning.utilities.apply_to_collection` ([#7433](https://github.com/PyTorchLightning/pytorch-lightning/pull/7433))
 
 
+- Fixed setting correct `DistribType` for `ddp_cpu` (spawn) backend ([#7492](https://github.com/PyTorchLightning/pytorch-lightning/pull/7492))
+
 
 ## [1.3.1] - 2021-05-11