Fix AutoModel not loading model correctly due to config_class inconsistency

This fixes an issue when using AutoModel to instantiate the model where the config class instantiated with the model is from the transformers library instead of the model's module. This causes the instantiation to fail with the error below. See [this Github issue](https://github.com/huggingface/transformers/issues/31068) for more details.
```
Traceback (most recent call last):
model = AutoModel.from_pretrained("zhihan1996/DNABERT-2-117M", trust_remote_code=True)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".../lib/python3.11/site-packages/transformers/models/auto/auto_factory.py", line 560, in from_pretrained
cls.register(config.__class__, model_class, exist_ok=True)
File ".../lib/python3.11/site-packages/transformers/models/auto/auto_factory.py", line 586, in register
raise ValueError(
ValueError: The model class you are passing has a `config_class` attribute that is not consistent with the config class you passed (model has <class 'transformers.models.bert.configuration_bert.BertConfig'> and you passed <class 'transformers_modules.zhihan1996.DNABERT-2-117M.d064dece8a8b41d9fb8729fbe3435278786931f1.configuration_bert.BertConfig'>. Fix one of those so they match!
```

bert_layers.py

@@ -24,6 +24,7 @@ from transformers.modeling_utils import PreTrainedModel
 from .bert_padding import (index_first_axis,
                                             index_put_first_axis, pad_input,
                                             unpad_input, unpad_input_only)
+from .configuration_bert import BertConfig
 
 try:
     from .flash_attn_triton import flash_attn_qkvpacked_func
@@ -564,7 +565,8 @@ class BertModel(BertPreTrainedModel):
     all_encoder_layers, pooled_output = model(input_ids, token_type_ids, input_mask)
     ```
     """
-
+    config_class = BertConfig
+    
     def __init__(self, config, add_pooling_layer=True):
         super(BertModel, self).__init__(config)
         self.embeddings = BertEmbeddings(config)