ddp_backend in 0.9.0rc12 fails if no CUDA_VISIBLE_DEVICES found
🐛 Bug
In ddp_backend, training immediately fails if the environment variable CUDA_VISIBLE_DEVICES isn't set. This line should handle the None case gracefully: https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/accelerators/ddp_backend.py#L90
To Reproduce
Start a run using ddp on CPU. This was discovered using torchelastic to launch
Code sample

Expected behavior
This shouldn't crash if the environment variable isn't set. We could default to num_gpus = 0 in this case.
Replacing the line above with something like this could work:
num_gpus = os.environ.get('CUDA_VISIBLE_DEVICES', []).split(',').__len__()