Temp fix for broken detectron2 import
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Detectron2 is currently broken on master as can be seen here:
- https://github.com/facebookresearch/detectron2/issues/4489
- https://github.com/facebookresearch/detectron2/issues/4487

In order to not affect all kinds of tests, we add a temporary fix here that will only let `LayoutLMv2` fail that is dependent on `detectron2`, but other tests that just import the layoutlmv2 module won't be affected so that the Circle CI stays green


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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Hi @patrickvonplaten . Please review the thread at https://github.com/facebookresearch/detectron2/issues/4489 and let me know if the landed change means we can now revert this temporary fix. Thanks.