Update CLIPFeatureExtractor to convert PIL image to RGB
Currently PIL images with RGBA format throws an error when being processed by CLIPFeatureExtractor.

CLIPFeatureExtractor.normalize() throws the following error:
.../sentence-transformer/lib/python3.9/site-packages/transformers/image_utils.py", line 185, in normalize return (image - mean) / std ValueError: operands could not be broadcast together with shapes (4,224,224) (3,)

The original [clip model preprocesses PIL Images](https://github.com/openai/CLIP/blob/main/clip/clip.py#L74) by converting all PIL images into RGB format.
_The documentation is not available anymore as the PR was closed or merged._<|||||>hi @patil-suraj, re-created the PR as discussed 😄