Trainer: fix support for non-distributed PyTorch (#14971)

* Trainer: fix non-distributed use
* Update CHANGELOG

Author: adamjstewart

src/pytorch_lightning/CHANGELOG.md

@@ -169,6 +169,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed an issue with terminating the trainer profiler when a `StopIteration` exception is raised while using an `IterableDataset` ([#14940](https://github.com/Lightning-AI/lightning/pull/14945))
 
 
+- Fixed `Trainer` support for PyTorch built without distributed support ([#14971](https://github.com/Lightning-AI/lightning/pull/14971))
+
+
 
 ## [1.7.7] - 2022-09-22