Add Image To Text Generation pipeline (#18821)

* Add Image2TextGenerationPipeline to supported pipelines

* Add Flax and Tensorflow support

* Add Flax and Tensorflow small tests

* Add default model for Tensorflow

* Add docstring

* Fix doc style

* Add tiny models for pytorch and flax

* Remove flax from pipeline.
Fix tests

* Use ydshieh/vit-gpt2-coco-en as a default for both PyTorch and Tensorflow

* Fix Tensorflow support

Co-authored-by: Olivier Dehaene <olivier@huggingface.co>

docs/source/en/main_classes/pipelines.mdx

@@ -29,6 +29,7 @@ There are two categories of pipeline abstractions to be aware about:
   - [`FillMaskPipeline`]
   - [`ImageClassificationPipeline`]
   - [`ImageSegmentationPipeline`]
+  - [`Image2TextGenerationPipeline`]
   - [`ObjectDetectionPipeline`]
   - [`QuestionAnsweringPipeline`]
   - [`SummarizationPipeline`]
@@ -365,6 +366,12 @@ That should enable you to do all the custom code you want.
     - __call__
     - all
 
+### Image2TextGenerationPipeline
+
+[[autodoc]] Image2TextGenerationPipeline
+    - __call__
+    - all
+
 ### NerPipeline
 
 [[autodoc]] NerPipeline