Add support for multiple processor models in the Processor class

It could be interesting to support the idea of a multi-controlnet or Uni-controlnet by adding support in the `Processor` class for loading multiple models at once. Or we can shift the functionality to a `MultiProcessor` class.

```python
class MultiProcessor:
def __init__(self, processor_id: Union[List[str], str]):
# if processor_id is a string, load one processor
# if it's a list of strings, load multiple processors

def __call__(self, image: Union[List[Image.Image], Image.Image]):
# if we get one image, put the image through all the processors
# if we get a list of images, put each image through the according processor
# assuming that the list of images is the same length of the list of processors
# this could be used for a multicontrolnet where we want to have conditionings starting
# from various source images
```