Finetune BLIP on customer dataset
### System Info

Dear the team,

I was trying to finetune BLIP and so far I got an error, not sure how to solve it. Is it possible that you can give me some advice? Thanks



### Who can help?

_No response_

### Information

- [X] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

```
from PIL import Image
import requests
from transformers import BlipProcessor, BlipForQuestionAnswering

model = BlipForQuestionAnswering.from_pretrained("Salesforce/blip-vqa-base")
processor = BlipProcessor.from_pretrained("Salesforce/blip-vqa-base")

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
model.to(device)

import torch
from PIL import Image

class VQADataset(torch.utils.data.Dataset):
    """VQA (v2) dataset."""

    def __init__(self, questions, answers, image_paths, processor):
        self.questions = questions
        self.answers = answers
        self.image_paths = image_paths
        self.processor = processor

    def __len__(self):
        return len(self.questions)

    def __getitem__(self, idx):
        # get image + text
        question = self.questions[idx]
        answer = self.answers[idx]
        image = Image.open(self.image_paths[idx]).convert("RGB")
        text = question
        
        encoding = self.processor(image, text, padding="max_length", truncation=True, return_tensors="pt")
        labels = self.processor.tokenizer.encode(
            answer, max_length= 512, pad_to_max_length=True, return_tensors='pt'
        )
        encoding["labels"] = labels

        # remove batch dimension
        # for k,v in encoding.items():  encoding[k] = v.squeeze()
        return encoding

from torch.utils.data import DataLoader
from tqdm import tqdm

def collate_fn(batch):
    input_ids = [item['input_ids'] for item in batch]
    pixel_values = [item['pixel_values'] for item in batch]
    attention_mask = [item['attention_mask'] for item in batch]
    labels = [item['labels'] for item in batch]
    
    return batch


questions = list of questions 
answers = list of corresponding answers
image_paths = list of paths of corresponding images

train_dataset = VQADataset(questions = questions,
                          answers = answers,
                          image_paths = images,
                          processor=processor)

test_dataset = VQADataset(questions = questions,
                          answers = answers,
                          image_paths = images,
                          processor=processor)

batch_size = 1
train_dataloader = DataLoader(train_dataset, collate_fn=collate_fn, batch_size=batch_size, shuffle=False)
test_dataloader = DataLoader(test_dataset, collate_fn=collate_fn, batch_size=batch_size, shuffle=False)

batch = next(iter(train_dataloader))

print(batch[0].keys()) # dict_keys(['pixel_values', 'input_ids', 'attention_mask', 'labels'])

import copy 
test_input = copy.copy(batch[0]).to(device)
outputs = model(**test_input)
```

Example of the input:
```
questions = ["How many cats are there?"]
answers = ["two"]
image_paths = ["./img_125.png"]
```

### Expected behavior

```
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
[<ipython-input-27-f4758beea430>](https://localhost:8080/#) in <module>
      2 
      3 test_input = copy.copy(batch[0]).to(device)
----> 4 outputs = model(**test_input)

6 frames
[/usr/local/lib/python3.8/dist-packages/torch/nn/functional.py](https://localhost:8080/#) in cross_entropy(input, target, weight, size_average, ignore_index, reduce, reduction, label_smoothing)
   3024     if size_average is not None or reduce is not None:
   3025         reduction = _Reduction.legacy_get_string(size_average, reduce)
-> 3026     return torch._C._nn.cross_entropy_loss(input, target, weight, _Reduction.get_enum(reduction), ignore_index, label_smoothing)
   3027 
   3028 

ValueError: Expected input batch_size (0) to match target batch_size (511).
Please use the [forums](https://discuss.huggingface.co/) to help debug your code as we keep issues for bugs and feature requests only.<|||||>hi @dxlong2000 
Thanks for the issue! 
Can you open an issue on the forums as suggested by @sgugger , and ping me there? (my handle on the forum is @ybelkada) As I am interested in this question and help you
Thanks!<|||||>Did I tag you in the correct way @younesbelkada?
You can check here: https://discuss.huggingface.co/t/finetune-blip-on-customer-dataset-20893/28446/2

Thanks @sgugger!

<|||||>Thanks I can see the issue now! <|||||>Ok I close the issue now!