Fix NameError `init_empty_weights` when importing Blip2Processor
## Context 
Was trying to run [https://huggingface.co/Salesforce/blip2-flan-t5-xl](https://huggingface.co/Salesforce/blip-image-captioning-base) in a colab notebook (after installing `accelerate`) and was getting an error when importing Blip2Processor:
```
NameError `init_empty_weights` is not defined
```
<img width="1285" alt="Screen Shot 2023-04-01 at 1 31 17 pm" src="https://user-images.githubusercontent.com/3723005/229265367-36eed050-b75b-4d06-b69b-f80a649e2d2e.png">

## Changes
Splitting out the if statements to check for `accelerate` and `bitsandbytes` separately seems to fix this problem, in `load_in_8bit` mode.

Applies a similar fix to the imports in `deepspeed.py`. This fixed the same error as above but in this conditional block, later in the file.
```
    if is_deepspeed_zero3_enabled():
        import deepspeed

        [logger.info](http://logger.info/)("Detected DeepSpeed ZeRO-3: activating zero.init() for this model")
        init_contexts = [deepspeed.zero.Init(config_dict_or_path=deepspeed_config())] + init_contexts
```

After these changes, I was able to import, Blip2Processor, Blip2ForConditionalGeneration on Colab :)
@pacman100 Is this something you'd be able to review for me? This my first contribution in this repo but I'm pretty sure it fixes a real issue in loading BLIP2.<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Perhaps @NielsRogge, since this is your implementation of BLIP2, you could review for me? I was able to reproduce the error in colab.<|||||>Thanks for the review @sgugger, makes sense now