Cannot inherit from BertPretrainedModel  anymore after migrating to pytorch-transformers
Hi, 

After I updated my environment today, I cannot run my old code anymore. I think I followed all the steps in migration section of README but still the following code gives me the `NameError: name 'BertPreTrainedModel' is not defined` error. To migrate latest version, I cloned the repository and run `pip install --editable .` command within the directory.

Here is the code: 

```python

from pytorch_transformers import *
class BertForMultiLabelSequenceClassification(BertPreTrainedModel):
    def __init__(self, config, num_labels=2):
        super(BertForMultiLabelSequenceClassification, self).__init__(config)
        self.num_labels = num_labels
        self.bert = BertModel("bert-base-multilingual-cased")       
        self.dropout = torch.nn.Dropout(config.hidden_dropout_prob)
        self.classifier = torch.nn.Linear(config.hidden_size, num_labels)
        self.apply(self.init_bert_weights)

    def forward(self, input_ids, token_type_ids=None, attention_mask=None, labels=None):
        _, pooled_output = self.bert(input_ids, token_type_ids, attention_mask, output_all_encoded_layers=False)
        pooled_output = outputs[-1]
        pooled_output = self.dropout(pooled_output)
        logits = self.classifier(pooled_output)
        return logits

args = {
    "train_size": -1,
    "val_size": -1,
    "bert_model": "bert-base-multilingual-cased",
    "do_lower_case":False,
    "max_seq_length": 100,
    "do_train": True,
    "do_eval": True,
    "train_batch_size": 32,
    "eval_batch_size": 32,
    "learning_rate": 3e-5,
    "num_train_epochs": 20,
    "warmup_proportion": 0.1,
    "no_cuda": False,
    "local_rank": -1,
    "seed": 42,
}

num_labels = 2
model = BertForMultiLabelSequenceClassification.from_pretrained(args['bert-model'],num_labels)
```
You should do `from pytorch_transformers.modeling_bert import BertPreTrainedModel`

I'll add these to the main `__init__.py`<|||||>Thank you for the answer @thomwolf . It solved that error but now I'm getting another one (which wasn't there when I was using previous versions of the repository): `TypeError: unhashable type: 'BertConfig'` what could be wrong ?  <|||||>We need a full error log and more details.