Support `padding_side` in `Blip2Processor`
### Feature request

Support different `padding_side` for `Blip2Processor`.

### Motivation

When I use BLIP2 LLM, I found that the padding style is different between decoder-only model (opt-2.7b for example) and encoder-decoder model (flan-t5-xl for example). So I assume that the paddings are different from `Salesforce/blip2-opt-2.7b` and `Salesforce/blip2-flan-t5-xl`. But actually I got the same padding results, the default is `padding_side`=right. 

Code example:
```
prompt = ["hello world", "Question: how many cats are there? Answer:"]
processor_1 = Blip2Processor.from_pretrained("Salesforce/blip2-opt-2.7b")
processor_2 = Blip2Processor.from_pretrained("Salesforce/blip2-flan-t5-xl")

inputs_1 = processor_1(text=prompt, return_tensors="pt", padding=True)
inputs_2 = processor_2(text=prompt, return_tensors="pt", padding=True)
```

Output (the same for inputs_1 and inputs_2):
```
{'input_ids': tensor([[    2, 42891,   232,     1,     1,     1,     1,     1,     1,     1,
             1],
        [    2, 45641,    35,   141,   171, 10017,    32,    89,   116, 31652,
            35]]), 'attention_mask': tensor([[1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0],
        [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]])}
```

I believe that right padding for flan-t5 will give the wrong outputs when calling `generate`, correct me if I am wrong: (transformers/generation/utils.py)
<img width="940" alt="image" src="https://user-images.githubusercontent.com/16580382/223613908-3ca57575-5536-4296-a449-e49ad3b4fa90.png">


Expected outputs (when setting `padding_side`=left):
```
{'input_ids': tensor([[    1,     1,     1,     1,     1,     1,     1,
             1,     2, 42891,   232],
        [    2, 45641,    35,   141,   171, 10017,    32,    89,   116, 31652,
            35]]), 'attention_mask': tensor([[0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]])}
```

### Your contribution

I found the this `padding_side` feature exists in `AutoTokenizer`, is that possible to move this feature in `Blip2Processor`? 
Hi,

You can achieve that by simply updating the `padding_side` attribute of the processor's tokenizer:
```
processor.tokenizer.padding_side = "left"
```
Note that Blip2Processor is just a wrapper around both the image processor and the tokenizer.<|||||>Thanks a lot!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.