Add "Leveraging Pretrained Checkpoints for Generation" Seq2Seq models. (#6594)

* add conversion script

* improve conversion script

* make style

* add tryout files

* fix

* update

* add causal bert

* better names

* add tokenizer file as well

* finish causal_bert

* fix small bugs

* improve generate

* change naming

* renaming

* renaming

* renaming

* remove leftover files

* clean files

* add fix tokenizer

* finalize

* correct slow test

* update docs

* small fixes

* fix link

* adapt check repo

* apply sams and sylvains recommendations

* fix import

* implement Lysandres recommendations

* fix logger warn

src/transformers/generation_utils.py

@@ -383,7 +383,11 @@ class GenerationMixin:
                 # see if BOS token can be used for decoder_start_token_id
                 if bos_token_id is not None:
                     decoder_start_token_id = bos_token_id
-                elif hasattr(self.config, "decoder") and hasattr(self.config.decoder, "bos_token_id"):
+                elif (
+                    hasattr(self.config, "decoder")
+                    and hasattr(self.config.decoder, "bos_token_id")
+                    and self.config.decoder.bos_token_id is not None
+                ):
                     decoder_start_token_id = self.config.decoder.bos_token_id
                 else:
                     raise ValueError(