add changelog

Author: awaelchli

CHANGELOG.md

@@ -44,6 +44,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Changed `automatic casting` for LoggerConnector `metrics` ([#5218](https://github.com/PyTorchLightning/pytorch-lightning/pull/5218))
 
 
+- Changed the default of `find_unused_parameters` to `False` in DDP ([#5185](https://github.com/PyTorchLightning/pytorch-lightning/pull/5185))
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 - `stat_scores` metric now calculates stat scores over all classes and gains new parameters, in line with the new `StatScores` metric ([#4839](https://github.com/PyTorchLightning/pytorch-lightning/pull/4839))