enable loading Mbart50Tokenizer with AutoTokenizer 
# What does this PR do?


Currently `MBart50Tokenizer`, `MBart50TokenizerFast` can't be loaded using `AutoTokenizer` because they use the `MBartConfig` which is associated with `MBartTokenizer`. 

This PR enables loading `MBart50Tokenizer(Fast)` by adding them to the `NO_CONFIG_TOKENIZER` list. I've also added the `tokenizer_type` argument in the respective models' config file on the hub.

cc @Narsil 
Very nice ! 