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

* Trainer: fix non-distributed use
* Update CHANGELOG

Author: adamjstewart

src/pytorch_lightning/CHANGELOG.md

@@ -307,6 +307,9 @@ 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
 

src/pytorch_lightning/trainer/trainer.py

@@ -2233,7 +2233,7 @@ def _evaluation_context(accelerator: Accelerator) -> Generator:
     # and HPU & TPU accelerators.
     context_manager_class = (
         torch.inference_mode
-        if not (dist.is_initialized() and dist.get_backend() == "gloo")
+        if not (dist.is_available() and dist.is_initialized() and dist.get_backend() == "gloo")
         and not isinstance(accelerator, HPUAccelerator)
         and not isinstance(accelerator, TPUAccelerator)
         else torch.no_grad