ddp with subprocess does not work with hydra

[haichao592]

Thanks for the great work.

The recent change replaced .spawn with subprocess which forces pass "--gpus" flag. This works fine for the project using argparse to parse arguments. But it does not work with hydra since the arguments in hydra are passed like "gpus=2" not "--gpus 2". The "--somearg val" in hydra is used for specific purpose not for arguments passing.

Could you please provide the support for hydra under the new context?
Thanks.

https://github.com/PyTorchLightning/pytorch-lightning/blob/82a20296e308a67c8d9202e4cbdf92a44b90b077/pytorch_lightning/trainer/distrib_data_parallel.py#L370-L372

[github-actions]

Hi! thanks for your contribution!, great first issue!