Fix Segmentation faults for GPU installation

[jannisborn]

Critically, segmentation faults upon import pytorch_lightning can be prevented when torch and tensorflow are imported in the right oder:

import torch
import tensorflow
import pytorch_lightning

This cannot be enforced with isort at the moment, because it cannot force multiple packages to the top of the import lists while respecting a non-alphabetic ordering. Hence, isort is disabled from the test suite until a better fix is available