Avoid modeling tests run in pipeline CI jobs
# What does this PR do?

Avoid modeling tests run in pipeline CI jobs.


PR #21887 applied
```
@is_pipeline_test
class PipelineTesterMixin:
```
Together the changes in #21516
```
class BertModelTest(ModelTesterMixin, PipelineTesterMixin, unittest.TestCase):
```
these 2 make all modeling test methods become pipeline tests, therefore CircleCI pipeline jobs run all pipeline tests + all model tests. This causes OOM as pipeline jobs are run with `-n 8`.

This PR applies `@is_pipeline_test` to each test methods instead of the test class to avoid such issue.


### Effect

Run
```python
python -m pytest -m is_pipeline_test -v tests/models/bridgetower/test_modeling_bridgetower.py::BridgeTowerModelTest::test_model
```

#### On main
test pass

#### on this PR
test deselected
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_21911). All of your documentation changes will be reflected on that endpoint.