Auto feature extractor (#11097)

* AutoFeatureExtractor

* Init and first tests

* Tests

* Damn you gitignore

* Quality

* Defensive test for when not all backends are here

* Use pattern for Speech2Text models

tests/fixtures/dummy_feature_extractor_config.json

@@ -0,0 +1,3 @@
+{
+  "feature_extractor_type": "Wav2Vec2FeatureExtractor"
+}