[WavLM] Layerdrop is not allowed for first layer
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes flaky CI. WavLM makes use of relative position encoding that are **always** computed in the first attention layer and then passed down to subsequent layers. Hence the first layer **cannot** be skipped when using layerdrop.

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Comments:
Thanks for fixing!