1D tensors fail

* sparsemax version: pip latest
* Python version: 3.8
* Operating System: Ubuntu 20.04

### Description
Sparsemax fails for 1D tensors

### What I Did

```
>>> import torch
>>> from sparsemax import Sparsemax
>>> Sparsemax()(torch.rand(10))

RuntimeError: expand(torch.LongTensor....
```

