Low accuracy after load custom pretrained model in a text binary classification problem
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.4.0
- Platform: Linux-4.15.0-122-generic-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.5.1+cpu (False)
- Tensorflow version (GPU?): 2.3.0 (False)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: Distributed (not really sure)

### Who can help
@LysandreJik

## Information
Posted in StackOverflow. Received a comment with two similar issues regarding save and load custom models. The original question can be found at: https://stackoverflow.com/questions/64666510/huggingface-transformers-low-accuracy-after-load-custom-pretrained-model-in-a-t?noredirect=1#comment114344159_64666510

In a nutshell I am using BertForSequenceClassification (PyTorch) with ```dccuchile/bert-base-spanish-wwm-cased``` for solving a binary classification problem. I have trained the network and evaluate the model with a testing dataset (different from the training dataset). I have achieved an ```acc``` and ```val_acc``` between 0.85 and 0.9. However, after I save the model and retrieve it again in another script, the accuracy is similar to a random classifier (0.41).

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [X] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [X] my own task or dataset: (give details below)

## To reproduce
This is the code I am using for training and evaluating (during training):
```
criterion = torch.nn.CrossEntropyLoss ()
criterion = criterion.to (device)
optimizer = AdamW (model.parameters(), lr=5e-5)

for epoch in range (4):

    i = 0

    # Train this epoch
    model.train ()
    for batch in train_loader:
        optimizer.zero_grad ()
        input_ids = batch['input_ids'].to (device)
        attention_mask = batch['attention_mask'].to (device)
        labels = batch['label'].to (device)

        loss, _ = model (input_ids, attention_mask=attention_mask, labels=labels)
        _, preds = torch.max (_, dim=1)
        correct_predictions += torch.sum (preds == labels)
        i += 1
        acc = correct_predictions.item ()  / (batch_size * i)
        
        loss.backward ()
        optimizer.step ()
    
    
    # Eval this epoch with the testing dataset
    model = model.eval ()
    correct_predictions = 0
    with torch.no_grad ():
        for batch in test_loader:
            input_ids = batch['input_ids'].to (device)
            attention_mask = batch['attention_mask'].to (device)
            labels = batch['label'].to (device)

            loss, _ = model (input_ids, attention_mask=attention_mask, labels=labels)
            _, preds = torch.max (_, dim=1)
            correct_predictions += torch.sum (preds == labels)

model.bert.save_pretrained ("my-model")
tokenizer.save_pretrained ("my-model")
```

After this step, I got good accuracy after the first epoch

Then, I load the model again in another script
```
model = BertForSequenceClassification.from_pretrained ("my-model")

# Eval this epoch with the testing dataset
model = model.eval ()
correct_predictions = 0
with torch.no_grad ():
    for batch in test_loader:
        input_ids = batch['input_ids'].to (device)
        attention_mask = batch['attention_mask'].to (device)
        labels = batch['label'].to (device)

        loss, _ = model (input_ids, attention_mask=attention_mask, labels=labels)
        _, preds = torch.max (_, dim=1)
        correct_predictions += torch.sum (preds == labels)

print (correct_predictions.item () / len (test_df))
```

but the accuracy is similar as If I retrieved a non-trained model. 

## Expected behavior
After load a model saved with ```save_pretrained```, the model should provide similar accuracy and loss for the same data.

The problem is mine. As other user suggest me in Stackoverflow, I have to save the model this way
```
model.save_pretrained ("my-model")`
```