Model Templates for Seq2Seq (#9251)

* adapt cookie cutter

* fix copy past statement

* delete copy statements for now

* remove unused import from template

* make doc rst

* correct config docstring

* correct training

* correct inputs processing tf enc dec

* make style

* adapt templates

* clean tabs

* correct tensor -> Tensor naming

* correct indent

* correct templates

* fix the test

* break lines to avoid > 119

* Apply suggestions from code review

templates/adding_a_new_model/tests/tf-encoder-bert-tokenizer.json

@@ -6,5 +6,6 @@
   "authors": "The HuggingFace Team",
   "checkpoint_identifier": "brand-new-bert-base-cased",
   "tokenizer_type": "Based on BERT",
-  "generate_tensorflow_and_pytorch": "TensorFlow"
+  "generate_tensorflow_and_pytorch": "TensorFlow",
+  "is_encoder_decoder_model": "False"
 }