Add OWL-ViT model for zero-shot object detection (#17938)

* add owlvit model skeleton

* add class and box predictor heads

* convert modified flax clip to pytorch

* fix box and class predictors

* add OwlViTImageTextEmbedder

* convert class and box head checkpoints

* convert image text embedder checkpoints

* add object detection head

* fix bugs

* update conversion script

* update conversion script

* fix q,v,k,out weight conversion conversion

* add owlvit object detection output

* fix bug in image embedder

* fix bugs in text embedder

* fix positional embeddings

* fix bug in inference mode vision pooling

* update docs, init tokenizer and processor files

* support batch processing

* add OwlViTProcessor

* remove merge conflicts

* readd owlvit imports

* fix bug in OwlViTProcessor imports

* fix bugs in processor

* update docs

* fix bugs in processor

* update owlvit docs

* add OwlViTFeatureExtractor

* style changes, add postprocess method to feature extractor

* add feature extractor and processor tests

* add object detection tests

* update conversion script

* update config paths

* update config paths

* fix configuration paths and bugs

* fix bugs in OwlViT tests

* add import checks to processor

* fix docs and minor issues

* fix docs and minor issues

* fix bugs and issues

* fix bugs and issues

* fix bugs and issues

* fix bugs and issues

* update docs and examples

* fix bugs and issues

* update conversion script, fix positional embeddings

* process 2D input ids, update tests

* fix style and quality issues

* update docs

* update docs and imports

* update OWL-ViT index.md

* fix bug in OwlViT feature ext tests

* fix code examples, return_dict by default

* return_dict by default

* minor fixes, add tests to processor

* small fixes

* add output_attentions arg to main model

* fix bugs

* remove output_hidden_states arg from main model

* update self.config variables

* add option to return last_hidden_states

* fix bug in config variables

* fix copied from statements

* fix small issues and bugs

* fix bugs

* fix bugs, support greyscale images

* run fixup

* update repo name

* merge OwlViTImageTextEmbedder with obj detection head

* fix merge conflict

* fix merge conflict

* make fixup

* fix bugs

* fix bugs

* add additional processor test