[tests] fix the wrong output in `ImageToTextPipelineTests.test_conditional_generation_llava`
Author: faaany
Pull request: yes

## What does this PR do?
Fix the below failing test: 
```bash
(dev) (base) [fanli@skyocean transformers]$ RUN_SLOW=1 pytest tests/pipelines/test_pipelines_image_to_text.py::Imag
eToTextPipelineTests::test_conditional_generation_llava -v -rA
=============================================== test session starts ===============================================
platform linux -- Python 3.10.13, pytest-7.4.4, pluggy-1.4.0 -- /home/fanli/.conda/envs/dev/bin/python
cachedir: .pytest_cache
rootdir: /mnt/disk4/fanlilin/transformers
configfile: pyproject.toml
plugins: anyio-4.2.0, xdist-3.5.0, timeout-2.3.1, env-1.1.3, excel-1.6.0
collected 1 item                                                                                                  

tests/pipelines/test_pipelines_image_to_text.py::ImageToTextPipelineTests::test_conditional_generation_llava FAILED [100%]
==================================================== FAILURES =====================================================
___________________________ ImageToTextPipelineTests.test_conditional_generation_llava ____________________________

self = <tests.pipelines.test_pipelines_image_to_text.ImageToTextPipelineTests testMethod=test_conditional_generation_llava>

    @slow
    @require_torch
    def test_conditional_generation_llava(self):
        pipe = pipeline("image-to-text", model="llava-hf/bakLlava-v1-hf")
    
        prompt = (
            "<image>\nUSER: What does the label 15 represent? (1) lava (2) core (3) tunnel (4) ash cloud?\nASSISTANT:"
        )
    
        outputs = pipe(
            "https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/transformers/tasks/ai2d-demo.jpg",
            prompt=prompt,
            generate_kwargs={"max_new_tokens": 200},
        )
>       self.assertEqual(
            outputs,
            [
                {
                    "generated_text": "<image> \nUSER: What does the label 15 represent? (1) lava (2) core (3) tunnel (4) ash cloud?\nASSISTANT: Lava"
                }
            ],
        )
E       AssertionError: Lists differ: [{'generated_text': '\nUSER: What does the label 15 represent?[59 chars]va'}] != [{'generated_text': '<image> \nUSER: What does the label 15 re[67 chars]va'}]
E       
E       First differing element 0:
E       {'generated_text': '\nUSER: What does the label 15 represent?[58 chars]ava'}
E       {'generated_text': '<image> \nUSER: What does the label 15 re[66 chars]ava'}
E       
E       - [{'generated_text': '\n'
E       + [{'generated_text': '<image> \n'
E       ?                      ++++++++
E       
E                             'USER: What does the label 15 represent? (1) lava (2) core '
E                             '(3) tunnel (4) ash cloud?\n'
E                             'ASSISTANT: Lava'}]

tests/pipelines/test_pipelines_image_to_text.py:289: AssertionError
---------------------------------------------- Captured stderr call -----------------------------------------------
Loading checkpoint shards: 100%|██████████| 4/4 [00:01<00:00,  2.07it/s]
Special tokens have been added in the vocabulary, make sure the associated word embeddings are fine-tuned or trained.
============================================= short test summary info =============================================
FAILED tests/pipelines/test_pipelines_image_to_text.py::ImageToTextPipelineTests::test_conditional_generation_llava - AssertionError: Lists differ: [{'generated_text': '\nUSER: What does the label 15 represent?[59 chars]va'}] !=...

```
The output of the Llava model doesn't contain the "<image>" label. You can checkout [the example](https://huggingface.co/llava-hf/bakLlava-v1-hf) in the model card as well. We should remove `<image>` and then the test passes:

```bash
=========================================================================== short test summary info ===========================================================================
PASSED tests/pipelines/test_pipelines_image_to_text.py::ImageToTextPipelineTests::test_conditional_generation_llava
``` 

## Who can review?
@younesbelkada @ArthurZucker 


Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_29975). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.