Fixes #2942 (#2969)

* Fixes #2942

* doc fix

docs/source/debugging.rst

@@ -1,8 +1,8 @@
 .. testsetup:: *
 
     from pytorch_lightning.trainer.trainer import Trainer
-    
- .. _debugging:
+
+.. _debugging:
 
 Debugging
 =========

pytorch_lightning/accelerators/ddp_backend.py

@@ -87,7 +87,7 @@ class DDPBackend(object):
         command = [sys.executable] + command
 
         # since this script sets the visible devices we replace the gpus flag with a number
-        num_gpus = os.environ['CUDA_VISIBLE_DEVICES'].split(',').__len__()
+        num_gpus = os.environ.get('CUDA_VISIBLE_DEVICES', []).split(',').__len__()
 
         if '--gpus' in command:
             gpu_flag_idx = command.index('--gpus')