update changelog

Author: tchaton

CHANGELOG.md

@@ -18,6 +18,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed `ModelPruning(make_pruning_permanent=True)` pruning buffers getting removed when saved during training ([#6073](https://github.com/PyTorchLightning/pytorch-lightning/pull/6073))
 
 
+- Fixed when `_stable_1d_sort` to work when `n >= N` ([#6177](https://github.com/PyTorchLightning/pytorch-lightning/pull/6177))
+
+
 - Fixed `AttributeError` when `logger=None` on TPU ([#6221](https://github.com/PyTorchLightning/pytorch-lightning/pull/6221))