Raise exceptions when torch distributed is not available (Lightning-A…

…I#10418)

* Raise exceptions when torch distributed is not avalible

* add changelog

CHANGELOG.md

@@ -25,7 +25,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Changed
 
--
+- Raise exception in `init_dist_connection()` when torch distibuted is not available ([#10418](https://github.com/PyTorchLightning/pytorch-lightning/issues/10418))
 
 
 -

pytorch_lightning/utilities/distributed.py

@@ -381,7 +381,9 @@ def init_dist_connection(
     world_size = world_size if world_size is not None else cluster_environment.world_size()
     os.environ["MASTER_ADDR"] = cluster_environment.main_address
     os.environ["MASTER_PORT"] = str(cluster_environment.main_port)
-    if torch.distributed.is_available() and not torch.distributed.is_initialized():
+    if not torch.distributed.is_available():
+        raise RuntimeError("torch.distributed is not available. Cannot initialize distributed process group")
+    if not torch.distributed.is_initialized():
         log.info(f"initializing distributed: GLOBAL_RANK: {global_rank}, MEMBER: {global_rank + 1}/{world_size}")
         torch.distributed.init_process_group(
             torch_distributed_backend, rank=global_rank, world_size=world_size, **kwargs