mBART generate random strings in end of sentence
### System Info

```shell
transformers==4.19.2
```


### Who can help?

@patil-suraj@patrickvonplaten, @Narsil, @gante

### Information

- [ ] The official example scripts
- [x] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [x] My own task or dataset (give details below)

### Reproduction

Since this happens to my personal project and the code is too custom, I just past the results and ```generete```call. As below show, sometimes mBART don't stop generating sentence properly but generate some strange tokens.
       
 ```
if "25" in self.hparams.model_name_or_path:
      generated_tokens = self.model.generate(input_ids=batch["input_ids"],
                                             attention_mask=batch["attention_mask"],
                                             decoder_start_token_id=self.tokenizer.lang_code_to_id[self.hparams.tgt_lang],
                                             do_sample=True,
                                             temperature=self.hparams.temperature,
                                             num_return_sequences=self.hparams.n_hypothesis,
                                             max_length=MAX_LENGTH)
  else:
      generated_tokens = self.model.generate(input_ids=batch["input_ids"],
                                             attention_mask=batch["attention_mask"],
                                             forced_bos_token_id=self.tokenizer.lang_code_to_id[self.hparams.tgt_lang],
                                             do_sample=True,
                                             temperature=self.hparams.temperature,
                                             num_return_sequences=self.hparams.n_hypothesis,
                                             max_length=MAX_LENGTH)
```
<img width="867" alt="屏幕快照 2022-06-15 上午10 40 25" src="https://user-images.githubusercontent.com/38279341/173725168-c867fc87-6f0f-4205-a76e-bf70b5a946d6.png">
<img width="412" alt="屏幕快照 2022-06-15 上午10 44 52" src="https://user-images.githubusercontent.com/38279341/173725722-271fb4d4-ccdc-47b0-be5c-31296c596fb2.png">



### Expected behavior

```shell
mBART all ways generate valid sentence.
```
