What does this PR do?

Addresses issue when importing functions in the image_transforms module if Pillow isn't installed. Namely:

• functions which don't require Pillow have errors when importing. This was because other objects were only imported if Pillow was available e.g. ChannelDimension. Resolved by rearranging imports.
• Unuseful error message hiding that the issue is with Pillow in the environment. Resolved by adding a vision_required decorator to allow for errors only on select functions.

Fixes #20627

This needs to be merged in before the transforms can be removed from the init in #20704

Snippet below shows new behaviour when running in an environment without Pillow installed:

>>> import numpy as np
>>> 
>>> # Show we can import both methods without issue
>>> from transformers.image_transforms import rescale
>>> from transformers.image_transforms import center_crop
>>> 
>>> img = np.random.randint(0, 256, (3, 244, 360))
>>> 
>>> # We can call rescale successfully without having Pillow
>>> rescale_img = rescale(img, 10)
>>> print(rescale_img.shape)
(3, 244, 360)

>>> # center_crop call raises error
>>> cropped_img = center_crop(img, (100, 100))
Traceback (most recent call last):
  File "/Users/amyroberts/code/transformers/../scripts/test_image_transforms_imports.py", line 10, in <module>
    cropped_img = center_crop(img, (100, 100))
  File "/Users/amyroberts/code/transformers/src/transformers/utils/import_utils.py", line 1073, in wrapper
    raise ImportError(f"Method `{func.__name__}` requires Pillow.")
ImportError: Method `center_crop` requires Pillow.

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