DDP breaks when `python` does not refer to the correct interpreter
🐛 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:

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


Additional context

The problem lies at 
  
    
      pytorch-lightning/pytorch_lightning/trainer/distrib_data_parallel.py
    
    
         Line 422
      in
      0697dd3
    
  
  
    

        
          
           command = ['python'] + command 
        
    
  

 The python command is hardcoded here. On many systems, python is a symlink to python2, or does not exist.