question: `init_process_group` not called when training on multiple-GPUs
context: Hi,
  I’m trying to train a model on 2 GPUs. I do this by specifying Trainer(..., gpus=2). ddp_spawn should automatically be selected for the method, but I instead get the following message + error:
  UserWarning: You requested multiple GPUs but did not specify a backend, e.g. `Trainer(
  accelerator="dp"|"ddp"|"ddp2")`. Setting `accelerator="ddp_spawn"` for you.
    'You requested multiple GPUs but did not specify a backend, e.g.'
  GPU available: True, used: True
  TPU available: False, using: 0 TPU cores
  Traceback (most recent call last):
   File "train.py", line 186, in <module>
      main(sys.argv[1:])
    File "train.py", line 173, in main
      print(f"Logs for this experiment are being saved to {trainer.log_dir}")
    File ".../pypi__pytorch_lightning_python3_deps/pytorch_lightning/trainer/properties.py", line 137, in log_dir
      dirpath = self.accelerator.broadcast(dirpath)
    File ".../pypi__pytorch_lightning_python3_deps/pytorch_lightning/accelerators/accelerator.py", line 436, in broadcast
      return self.training_type_plugin.broadcast(obj, src)
    File ".../pypi__pytorch_lightning_python3_deps/pytorch_lightning/plugins/training_type/ddp_spawn.py", line 275, in broadcast
      return self.dist.broadcast(obj)
    File ".../pypi__pytorch_lightning_python3_deps/pytorch_lightning/distributed/dist.py", line 33, in broadcast
      broadcast_object_list(obj, 0, group=group or _group.WORLD)
    File ".../pypi__torch_python3_deps/torch/distributed/distributed_c10d.py", line 1700, in broadcast_object_list
      my_rank = get_rank()
    File ".../pypi__torch_python3_deps/torch/distributed/distributed_c10d.py", line 725, in get_rank
      default_pg = _get_default_group()
    File ".../pypi__torch_python3_deps/torch/distributed/distributed_c10d.py", line 358, in _get_default_group
      raise RuntimeError("Default process group has not been initialized, "
  RuntimeError: Default process group has not been initialized, please make sure to call init_process_group.
  
  I looked at the source code of ddp_spawn and it looks like it should print out a message when initializing ddp, but it didn’t.
  Could I please have advice on how to correct this error.
  Thank you!
answer: The issue comes from the line
    File "train.py", line 173, in main
      print(f"Logs for this experiment are being saved to {trainer.log_dir}")
  which tries to access trainer.log_dir outside of the trainer scope.
  trainer.log_dir tries to broadcast the directory but fails as DDP hasn’t been initialized yet.
    File ".../pypi__pytorch_lightning_python3_deps/pytorch_lightning/trainer/properties.py", line 137, in log_dir
      dirpath = self.accelerator.broadcast(dirpath)
  This is fixed in the 1.4 release as broadcast becomes a no-op in that case