[Marian] documentation and AutoModel support (#4152)

- MarianSentencepieceTokenizer - > MarianTokenizer
- Start using unk token.
- add docs page
- add better generation params to MarianConfig
- more conversion utilities

src/transformers/__init__.py

@@ -248,7 +248,7 @@ if is_torch_available():
         BART_PRETRAINED_MODEL_ARCHIVE_MAP,
     )
     from .modeling_marian import MarianMTModel
-    from .tokenization_marian import MarianSentencePieceTokenizer
+    from .tokenization_marian import MarianTokenizer
     from .modeling_roberta import (
         RobertaForMaskedLM,
         RobertaModel,