Single node DDP: "Default process group is not initialized"
🐛 Bug
Unable to start single node ddp training on 0.8.0
To Reproduce
was going to run the gpu_template but... #2235
both methods of running the template result in the same error
$ python -m pl_examples.basic_examples.gpu_template --gpus 4 --distributed_backend ddp_spawn
$ python -m pl_examples.basic_examples.gpu_template --gpus 4 --distributed_backend ddp

GPU available: True, used: True
TPU available: False, using: 0 TPU cores
CUDA_VISIBLE_DEVICES: [0,1,2,3]
Traceback (most recent call last):
  File "/opt/conda/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/opt/conda/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/opt/conda/lib/python3.6/site-packages/pl_examples/basic_examples/gpu_template.py", line 80, in <module>
    main(hyperparams)
  File "/opt/conda/lib/python3.6/site-packages/pl_examples/basic_examples/gpu_template.py", line 41, in main
    trainer.fit(model)
  File "/opt/conda/lib/python3.6/site-packages/pytorch_lightning/trainer/trainer.py", line 860, in fit
    self.barrier('fit_prepare_data')
  File "/opt/conda/lib/python3.6/site-packages/pytorch_lightning/trainer/trainer.py", line 1261, in barrier
    torch_distrib.barrier()
  File "/opt/conda/lib/python3.6/site-packages/torch/distributed/distributed_c10d.py", line 1484, in barrier
    _check_default_pg()
  File "/opt/conda/lib/python3.6/site-packages/torch/distributed/distributed_c10d.py", line 187, in _check_default_pg
    "Default process group is not initialized"
AssertionError: Default process group is not initialized