Unify BERT model API in one class (#387)

* Initial commit

* Fix model loading from config

* Fix vocab size typo and tests

* Add TODOs

* Fix docstrings and format

* Fix comments

* Address comments

* Use kwargs

* Attach presets to class with write protection

* Change preset accessor to `deepcopy`

* Remove redundant test

* Use `Bert.presets` to reference presets

* Remove more references to `backbone_presets`

keras_nlp/models/__init__.py

@@ -12,12 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from keras_nlp.models.bert.bert_models import BertBase
-from keras_nlp.models.bert.bert_models import BertCustom
-from keras_nlp.models.bert.bert_models import BertLarge
-from keras_nlp.models.bert.bert_models import BertMedium
-from keras_nlp.models.bert.bert_models import BertSmall
-from keras_nlp.models.bert.bert_models import BertTiny
+from keras_nlp.models.bert.bert_models import Bert
 from keras_nlp.models.bert.bert_preprocessing import BertPreprocessor
 from keras_nlp.models.bert.bert_tasks import BertClassifier
 from keras_nlp.models.distilbert.distilbert_models import DistilBertBase