Bug in DDP, but not DP modes.
Pytorch 1.5
In [3]: pytorch_lightning.__version__                                                                                                                                                                                                                                                                
Out[3]: '0.7.5rc1'


In DP everything works.
In DDP fails with:
 File "/home/vladimir/anaconda3/envs/solaris/lib/python3.7/multiprocessing/popen_fork.py", line 20, in __init__
    self._launch(process_obj)
  File "/home/vladimir/anaconda3/envs/solaris/lib/python3.7/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/home/vladimir/anaconda3/envs/solaris/lib/python3.7/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
_pickle.PicklingError: Can't pickle <class 'torch._C._VariableFunctions'>: it's not the same object as torch._C._VariableFunctions