Add the possibility to manually test on cuda devices

[schroedk]

To test the device handling of our torch related code, we should add the possibility to manually run tests with cuda
support.

Suggestion:

• Add --with-cuda option:

  parser.addoption(
          "--with-cuda",
          action="store_true",
          default=False,
          help="Use CUDA for testing if available",
      )

• Add a device fixture, which can be used in tests:

  @pytest.fixture(scope="session")
  def device(request):
      import torch
  
      use_cuda = request.config.getoption("--with-cuda")
      if use_cuda and torch.cuda.is_available():
          return torch.device("cuda")
      else:
          return torch.device("cpu")

• manually run pytest --with-cuda