[`ViTHybrid`] + [`BiT`] cleaner `__init__`
# What does this PR do?

a function in `BiT` is not needed, thus this PR makes the codebase less error-prone.
Before that, to get the feature maps size from the backbone model, `ViTHybrid` assumed that the backbone has the method `_get_feature_map` which is not the case for all backbones.

Related #20645
_The documentation is not available anymore as the PR was closed or merged._<|||||>Yes, this can be addressed in a future PR