Fix reduce-labels in BEIT Fast Image Processor
Author: simonreise
Pull request: yes

# What does this PR do?

When running `BeitImageProcessorFast` with `do_reduce_labels=True`, it not only reduces labels in segmentation map, but also applies `reduce_label` to the image itself

`reduce_label` should be applied only to segmentation maps

### Test

```py
import transformers
import torch


processor = transformers.BeitImageProcessorFast(
    do_resize=False, 
    do_center_crop=False, 
    do_rescale=False, 
    do_normalize=False, 
    do_convert_rgb=False,
)

image = torch.zeros([1, 3, 256, 256])

segmap = torch.zeros([1, 256, 256]) 

batch = processor.preprocess(
    images = image, 
    segmentation_maps = segmap, 
    return_tensors="pt", 
    do_reduce_labels=True,
)
```

pixel_values will be 255, should be 0.


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