new AutoModel classes in pipeline
The `translation`, `text-generation` and `fill-mask` pipelines still use the `AutoModelWithLMHead` class but from the warning it seems that it's going to be deprecated. Is there any specific reason for this or can we use the new `AutoModelForSeq2SeqLM` for translation, `AutoModelCausalLM` for text-generation and `AutoModelForMaskedLM` for fill-mask pipeline ? If yes then I'll be happy to open a PR.

@sgugger 