DDP breaks when python does not refer to the correct interpreter

[kl0211]

🐛 Bug

If using the DDP distributed_backend, the program breaks if python refers to python2 or does not exist.

To Reproduce

Steps to reproduce the behavior:

1. Make sure the python command does not link to python3, such as on Ubuntu 18.04.
2. Run Trainer().fit with distributed_backed='ddp'

Additional context

The problem lies at https://github.com/PyTorchLightning/pytorch-lightning/blob/0697dd306d578f0cd3c4d23b768da0106e54a095/pytorch_lightning/trainer/distrib_data_parallel.py#L422 The python command is hardcoded here. On many systems, python is a symlink to python2, or does not exist.

[github-actions]

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

[RitchieAlpha]

Same question!!!!!

[Borda]

it is based on your default python... @kl0211 @RitchieAlpha mind send a Pr which updating the actual python interpreter?
cc: @williamFalcon

[kl0211]

@Borda I just saw #2482. Using sys.executable is exactly what I was going to suggest. According to https://www.python.org/dev/peps/pep-0394/#for-python-script-publishers, this should ensure the same interpreter is used to spawn the child processes.

Thanks for the quick change!