🐛 Empty TypeError on BartTokenizerFast.decode(tensor)
## Environment info
`transformers` `3.0.2`

### Who can help
 Summarization: @sshleifer 

## To reproduce

```python
import torch
from transformers import BartTokenizerFast

t = BartTokenizerFast.from_pretrained('facebook/bart-large')

x = torch.tensor([0, 34, 45, 23, 54, 65, 765, 2])
t.decode(x)
```

will throw an empty `TypeError` :

```
File "/home/me/.venv/summarization/lib/python3.6/site-packages/tokenizers/implementations/base_tokenizer.py", line 267, in decode
    return self._tokenizer.decode(ids, skip_special_tokens=skip_special_tokens)
TypeError
```

To reproduce : [Colab Notebook](https://colab.research.google.com/drive/1bnP8TvmRrHrMD-7H2MOQQSE8QrhCb7SC?usp=sharing)

## Expected behavior

No error thrown, like with regular `Tokenizer` :

```python
import torch
from transformers import BartTokenizer

t = BartTokenizer.from_pretrained('facebook/bart-large')

x = torch.tensor([0, 34, 45, 23, 54, 65, 765, 2])
t.decode(x)
```

> `<s> has not at who one short</s>`
I noticed this is due to `x` being a tensor. It works fine with a list :

```python
import torch
from transformers import BartTokenizerFast

t = BartTokenizerFast.from_pretrained('facebook/bart-large')

x = [0, 34, 45, 23, 54, 65, 765, 2]
t.decode(x)
```

> `<s> has not at who one short</s>`

---

So the current work-around is to first convert to a list :

```python
import torch
from transformers import BartTokenizerFast

t = BartTokenizerFast.from_pretrained('facebook/bart-large')

x = torch.tensor([0, 34, 45, 23, 54, 65, 765, 2])
t.decode(x.tolist())
```<|||||>Tokenizer bug. @mfuntowicz is this expected behavior?<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
