skip `test_multi_gpu_data_parallel_forward` for `MaskFormerSwinModelTest`
# What does this PR do?

`MaskFormerSwinModel` outputs `hidden_states_spatial_dimensions`, which is ``tuple(tuple(int, int))``, and can't be collected by `nn.DataParallel` to form a final output value.

(When I remove this attribute, this test passes.)
_The documentation is not available anymore as the PR was closed or merged._