Support launching ddp job as module python -m ... 
🚀 Feature

Motivation
Some users wish to launch their training program as a module with python -m some.module.py
Pitch
We should evalute whether this is possible for ddp and support this option when possible.
We need to strip the -m argument and append it to the command with which we launch the child processes.
Alternatives

Additional context
This feature was orginally reported as a bug: #3600