WandbLogger cannot be used with 'ddp'
🐛 Bug
wandb modifies init such that a child process calling init returns None if the master process has called init. This seems to cause a bug with ddp, and results in rank zero having experiment = None, which crashes the program.
To Reproduce
Can be reproduced with the basic MNIST gpu template, simply add a WandbLogger and pass 'ddp' as the distributed backend.
-- Process 0 terminated with the following error:
Traceback (most recent call last):
  File "/home/rmrao/anaconda3/lib/python3.6/site-packages/torch/multiprocessing/spawn.py", line 19, in _wrap
    fn(i, *args)
  File "/home/rmrao/anaconda3/lib/python3.6/site-packages/pytorch_lightning/trainer/distrib_data_parallel.py", line 331, in ddp_train
    self.run_pretrain_routine(model)
  File "/home/rmrao/anaconda3/lib/python3.6/site-packages/pytorch_lightning/trainer/trainer.py", line 757, in run_pretrain_routine
    self.logger.log_hyperparams(ref_model.hparams)
  File "/home/rmrao/anaconda3/lib/python3.6/site-packages/pytorch_lightning/logging/base.py", line 14, in wrapped_fn
    fn(self, *args, **kwargs)
  File "/home/rmrao/anaconda3/lib/python3.6/site-packages/pytorch_lightning/logging/wandb.py", line 79, in log_hyperparams
    self.experiment.config.update(params)
AttributeError: 'NoneType' object has no attribute 'config'

This occurs with the latest wandb version and with pytorch-lightning 0.6.