text: Title: Allowing to switch start_method in Parallel executor
  Body: I have some Pytorch code which I want to run on GPU (`net.cuda()` and whatnot)
  
  I get the following error:
  ```
  RuntimeError: Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method
  ```
  I tried to fix this by either calling
  ```
  mp.set_start_method('spawn', force=True)
  ```
  
  or 
  ```
  torch.multiprocessing.set_start_method('spawn')
  ```
  
  In the first case nothing happens, and in the second I get `RuntimeError: context has already been set`
  
  How to set multiprocessing start method with ploomber?