XLNet tokenizer returns empty list instead of string for some indexes
## 🐛 Bug

<!-- Important information -->

Model I am using (Bert, XLNet....):

XLNet

Language I am using the model on (English, Chinese....):

English

The problem arise when using:
* [ ] my own modified scripts: (give details)

The tasks I am working on is:
* [ ] my own task or dataset: (give details)

## To Reproduce

Steps to reproduce the behavior:

1. Create tokenizer: tokenizer = XLNetTokenizer.from_pretrained(path)
2. Try to decode index 4: tokenizer.decode(4)
3. You get empty list [] although expected is a string

<!-- If you have a code sample, error messages, stack traces, please provide it here as well. -->

## Expected behavior

Decode method of the tokenizer should return a string and not a list.

## Environment

* OS:
* Python version: 3.7
* PyTorch version: 1.1.0
* PyTorch Transformers version (or branch): 1.2.0

Hi,
`tokenizer.decode()` expect a sequence of ids as indicated in the doc/docstring: https://huggingface.co/pytorch-transformers/main_classes/tokenizer.html#pytorch_transformers.PreTrainedTokenizer.decode<|||||>well, as far as i see, the tokenizer can accept both - sequence or single index. For example:
`tokenizer = XLNetTokenizer.from_pretrained('xlnet-base-cased')`
`tokenizer.decode(3)`
produce `'<cls>'` as well as `tokenizer.decode([3])`.

And the problem happens in both cases:
`tokenizer.decode([4])` produces empty string. And, what is even more strange, if index 4 is in the sequence, the result is a list instead of a string. For example:

`tokenizer.decode([35,109])` gives `'I new'`, but `tokenizer.decode([35,109,4])` generates list instead of a string `['I new']`<|||||>ok, it looks like 4 is an index of a separation token and it turns one sequence into sequence of sequences each of which will be decoded into string. So if index 4 presents in the sequence, the result will be list of strings. This behavior is quite unintuitive. If it was created purposely, should be properly documented. I've got very strange bug in my code that was hard to track, since index 4 was predicted very rarely and out of 500 predictions 1 was buggy.  <|||||>Thanks for the bug report. Indeed this was an unwanted behavior. Fixed on master now.