Add missing entries in mappings
# What does this PR do?

Add missing entries in mappings:  `TOKENIZER_MAPPING`, `FEATURE_EXTRACTOR_MAPPING`, `PROCESSOR_MAPPING`

The remaining models that don't have any `tokenizer/feature_extractor/processor` are:

- EncoderDecoder
- VisionEncoderDecoder
- SpeechEncoderDecoder
- DecisionTransformer

which are expected to have no processor.
_The documentation is not available anymore as the PR was closed or merged._<|||||>> * The `ImageGPT` changes should probably go in their own PR

Without the changes, the `test_pipeline` will fail for `ImageGPT`. Previously, `ImageGPT` was not used in pipeline testing because     it had no found tokenizer/feature_extractor.


I can add `("imagegpt", "ImageGPTFeatureExtractor")` in another PR instead of this PR, and make the necessary changes along with it. Let me know if this is necessary 😃 <|||||>Yes, I think ImageGPT should be added in a separate PR since it requires additional changes.<|||||>The changes on ImageGPT are now on a separate PR: #16869