[Marian] documentation and AutoModel support (#4152)

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

docs/source/model_doc/marian.rst

@@ -0,0 +1,43 @@
+MarianMTModel
+----------------------------------------------------
+**DISCLAIMER:** If you see something strange,
+file a `Github Issue <https://github.com/huggingface/transformers/issues/new?assignees=&labels=&template=bug-report.md&title>`__ and assign
+@sshleifer
+These models are for machine translation. The list of supported language pairs can be found `here <https://huggingface.co/Helsinki-NLP>`__.
+
+Opus Project
+~~~~~~~~~~~~
+The 1,000+ models were originally trained by `Jörg Tiedemann <https://researchportal.helsinki.fi/en/persons/j%C3%B6rg-tiedemann>`__ using the `Marian <https://marian-nmt.github.io/>`_ C++ library, which supports fast training and translation.
+All models are transformer encoder-decoders with 6 layers in each component. Each model's performance is documented in a model card.
+
+Implementation Notes
+~~~~~~~~~~~~~~~~~~~~
+- each model is about 298 MB on disk, there are 1,000+ models.
+- Models are named with the following patter 'Helsinki-NLP/opus-mt-{src_langs}-{targ_langs}'. If there are multiple source or target languages they are joined by a '+' symbol.
+- the 80 opus models that require BPE preprocessing are not supported.
+- There is an outstanding issue w.r.t multilingual models and language codes.
+- The modeling code is the same as ``BartModel`` with a few minor modifications:
+    - static (sinusoid) positional embeddings (``MarianConfig.static_position_embeddings=True``)
+    - a new final_logits_bias (``MarianConfig.add_bias_logits=True``)
+    - no layernorm_embedding (``MarianConfig.normalize_embedding=False``)
+    - the model starts generating with pad_token_id (which has 0 token_embedding) as the prefix. (Bart uses <s/>)
+- Code to bulk convert models can be found in ``convert_marian_to_pytorch.py``
+
+
+MarianMTModel
+~~~~~~~~~~~~~
+
+Pytorch version of marian-nmt's transformer.h (c++). Designed for the OPUS-NMT translation checkpoints.
+Model API is identical to BartForConditionalGeneration.
+Available models are listed at `Model List <https://huggingface.co/models?search=Helsinki-NLP>`__
+This class inherits all functionality from ``BartForConditionalGeneration``, see that page for method signatures.
+
+.. autoclass:: transformers.MarianMTModel
+    :members:
+
+
+MarianTokenizer
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.MarianTokenizer
+    :members: prepare_translation_batch