[`ImageProcessor`] Refactor default `mean` & `std` to `OPENAI_CLIP_MEAN` & `OPENAI_CLIP_STD`
# What does this PR do?

Initially this PR was intended to fix a small nit for BLIP feature extractors, the default normalization `mean` & `std` are currently incorrect as pointed out by @NielsRogge , BLIP uses the mean and std that are identical to CLIP: https://github.com/salesforce/LAVIS/blob/5ddd9b4e5149dbc514e81110e03d28458a754c5d/lavis/processors/blip_processors.py#L21 - this has no effect for the current models as the values were already correct on the Hub.

Therefore this PR adds a new variable in `constants.py`, `OPENAI_CLIP_MEAN` & `OPENAI_CLIP_STD` as this value is used by other models as well `CLIP`, `CLIPSeg`, `OWL-ViT` , etc.

cc @NielsRogge @sgugger 
_The documentation is not available anymore as the PR was closed or merged._