Fix Maskformer test
# What does this PR do?

Fix Maskformer test `test_multi_gpu_data_parallel_forward`.

Ignore the test, as the original workaround will break current checkpoints.

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Original Attempt

I know we probably want to avoid using `nn.DataParallel(model)`. But before doing so, I just tried my best to fix tests.
After spending some time debugging, I find using `add_module` instead of `nn.Sequential` causing problems.

I am not sure if the change in this PR is what we prefer though.

@NielsRogge Is there any reason to use `add_module`? I don't know if the comment regarding `Provide backwards compatibility ...` is really necessary.

https://github.com/huggingface/transformers/blob/d88719581b34f301edcc7772d927d8a3e3a77af6/src/transformers/models/maskformer/modeling_maskformer.py#L1986
_The documentation is not available anymore as the PR was closed or merged._<|||||>You are right, 

prev (and checkpoint on Hub): `model.pixel_level_module.decoder.fpn.stem.0.weight`

this PR: `model.pixel_level_module.decoder.fpn.stem.layers.1.weight` --> Get extra `layers` attribute


I will ignore the test<|||||>But maybe let's use `self.layers = nn.Sequential(...)` for future models in the 1st place?<|||||>Yes, the problem here is that the model inherited from `nn.Sequential`, which was a mistake. And to fix it, we had to go through this hacky way. But it's definitely not the recommended approach!<|||||>Test skipped now.