Fix model parallelism for `BridgeTower`
# What does this PR do?

Make `BridgeTower` work with model parallelism. The test `test_model_parallelism` is still skipped as tiny version hits edge cases.
With larger values of `hidden_size`, `num_hidden_layers` etc., it works now while failed before.
_The documentation is not available anymore as the PR was closed or merged._