Auto feature extractor
# What does this PR do?

This PR does a few things that are all tangled together to bring a new `AutoFeatureExtractor` API.

First it removes the check at init for `Speech2TextFeatureExtractor` to only import this object when the `speech` dep is installed and add a dummy class if it's not, so it's easier to add speech objects in the future (similar to what we did for vision) cc @patil-suraj

Then it adds a new `AutoFeatureExtractor` that works like `AutoConfig`: it picks the right class by either looking through the config to check if a `feature_extractor_type` is present then falls back to pattern matching on the name of the checkpoint (so it will work with existing checkpoint if their names contains the model code).


Really awesome! Thanks a lot Sylvain!