Training with DataParallel (DP) is broken
🐛 Bug
Currently, the distributed training backend DataParallel (DP) seems to be broken. Using DP will result in error
TypeError: zip argument #1 must support iteration. Below is the last few lines of the call stack:
  File "/home/ubuntu/anaconda3/envs/trfm/lib/python3.7/site-packages/pytorch_lightning/overrides/data_parallel.py", line 66, in forward
    return self.gather(outputs, self.output_device)
  File "/home/ubuntu/anaconda3/envs/trfm/lib/python3.7/site-packages/torch/nn/parallel/data_parallel.py", line 168, in gather
    return gather(outputs, output_device, dim=self.dim)
  File "/home/ubuntu/anaconda3/envs/trfm/lib/python3.7/site-packages/torch/nn/parallel/scatter_gather.py", line 68, in gather
    res = gather_map(outputs)
  File "/home/ubuntu/anaconda3/envs/trfm/lib/python3.7/site-packages/torch/nn/parallel/scatter_gather.py", line 62, in gather_map
    for k in out))
  File "/home/ubuntu/anaconda3/envs/trfm/lib/python3.7/site-packages/torch/nn/parallel/scatter_gather.py", line 62, in <genexpr>
    for k in out))
  File "/home/ubuntu/anaconda3/envs/trfm/lib/python3.7/site-packages/torch/nn/parallel/scatter_gather.py", line 63, in gather_map
    return type(out)(map(gather_map, zip(*outputs)))
TypeError: zip argument #1 must support iteration

To Reproduce
Run the following code will always result in this issue.
from pytorch_lightning import Trainer, seed_everything
from pl_examples.models.lightning_template import LightningTemplateModel

seed_everything(234)

def main():
    # model = LightningTemplateModel(**vars(args))
    model = LightningTemplateModel()

    trainer = Trainer(
        gpus=2,
        num_nodes=1,
        distributed_backend='dp',
        )

    trainer.fit(model)


if __name__ == '__main__':
    main()

Expected behavior
The trainer should initialize correctly and start training.
Environment

CUDA:
- GPU:
- Tesla V100-SXM2-16GB
- Tesla V100-SXM2-16GB
- Tesla V100-SXM2-16GB
- Tesla V100-SXM2-16GB
- available:         True
- version:           10.2
Packages:
- numpy:             1.18.5
- pyTorch_debug:     False
- pyTorch_version:   1.7.0.dev20200702
- pytorch-lightning: 0.8.5-dev
- tensorboard:       2.2.2
- tqdm:              4.47.0
System:
- OS:                Linux
- architecture:
- 64bit
-
- processor:         x86_64
- python:            3.7.7
- version:           #30~18.04.1-Ubuntu SMP Mon Jun 22 15:48:21 UTC 2020