[`ViTHybrid`] Fix `accelerate` slow tests
# What does this PR do?

Fixes failing `ViTHybrid` `accelerate` tests

Uses the same procedure as DPTHybrid for `backbone_featmap_shape` - Now all slow tests should pass :-) 

cc @sgugger @ydshieh 
_The documentation is not available anymore as the PR was closed or merged._