Allow custom code for Processors
# What does this PR do?

This PR allows code for dynamic processors, with the same API as configurations, feature extractors, models and tokenizers.

There needs to be a slight change in the way the `auto_map` field is stored in the dynamic tokenizers: we need to have a dict there like in the other configs. This PR changes the way new dynamic tokenizer configs are saved with backward compatibility to read the legacy formats. This is also checked with a test loading a dynamic tokenizer with the new and old format.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Very clean! Thanks