add changelog

Author: awaelchli

CHANGELOG.md

@@ -197,6 +197,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed torch distributed not available in setup hook for DDP ([#6506](https://github.com/PyTorchLightning/pytorch-lightning/pull/6506))
 
 
+- Fixed an issue with `IterableDataset` when `__len__` is not defined ([#6828](https://github.com/PyTorchLightning/pytorch-lightning/pull/6828))
+
+
 ## [1.2.6] - 2021-03-30
 
 ### Changed