[EncoderDecoder] google/roberta2roberta_L-24_wikisplit
It seems like `google/roberta2roberta_L-24_wikisplit` should be pre-processed differently than originally thought:

```python
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM

tokenizer = AutoTokenizer.from_pretrained("google/roberta2roberta_L-24_wikisplit")
model = AutoModelForSeq2SeqLM.from_pretrained("google/roberta2roberta_L-24_wikisplit")

long_sentence = """Due to the hurricane, Lobsterfest has been canceled, making Bob very happy about it and he decides to open Bob 's Burgers for customers who were planning on going to Lobsterfest."""

input_ids = tokenizer(long_sentence, return_tensors="pt").input_ids
output_ids = model.generate(input_ids)[0]
print(tokenizer.decode(output_ids, skip_special_tokens=True))
# should output
# Due Due hurricane, Lobsterfest has been canceled, making Bob very happy about it. He decides to open B
# ob's Burgers for customers who were planning on going to Lobsterfest.com.
```

yields a weird word duplication bug, whereas:

```python
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM

tokenizer = AutoTokenizer.from_pretrained("google/roberta2roberta_L-24_wikisplit")
model = AutoModelForSeq2SeqLM.from_pretrained("google/roberta2roberta_L-24_wikisplit")

long_sentence = """Due to the hurricane, Lobsterfest has been canceled, making Bob very happy about it and he decides to open Bob 's Burgers for customers who were planning on going to Lobsterfest."""

input_ids = tokenizer(tokenizer.bos_token + long_sentence + tokenizer.eos_token, return_tensors="pt").input_ids
output_ids = model.generate(input_ids)[0]
print(tokenizer.decode(output_ids, skip_special_tokens=True))
# should output
# Due to the hurricane, Lobsterfest has been canceled, making Bob very happy about it. He decides to open Bob's Burgers for customers who were planning on going to Lobsterfest.  
```

yields good results.
Notice the difference of 
`input_ids = tokenizer(tokenizer.bos_token + long_sentence + tokenizer.eos_token, return_tensors="pt").input_ids` between code sample 1 and 2.

@patrickvonplaten 

Wait for author's (@shashiongithub) answer before changing code example.

Confirmed by author that BOS and EOS have to be added -> fixed in https://github.com/huggingface/transformers/commit/0724c0f3a2d302246d0bd0b7d2f721fa902dee1b.