[Perceiver] Skip multi-gpu tests for now
Author: patrickvonplaten
Pull request: yes

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Multi-GPU for perceiver don't work due to deeply rooted problem in PyTorch's multi-gpu. 
For now I think we should skip the test. For the near future I think we should switch multi-gpu forward tests to DDP


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Comments:
Merging now as it's blocking the failing tests a bit. In a follow-up PR we should enable DDP as well