Fix a `BridgeTower` test
# What does this PR do?

A fix is required after #23029.

So far on main, an error is given

```bash
tests/models/bridgetower/test_modeling_bridgetower.py::BridgeTowerModelTrainingTest::test_training
(line 656)  AssertionError: unexpectedly None : Gradients should not be None - got None for bridgetower.cross_modal_image_layers.1.attention.self.query.weight
```
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the fix!