reduction is always 'mean'

Setting reduction = 'sum' does not work because of [this line](https://github.com/WeiChengTseng/Pytorch-PCGrad/blob/e987ac603fa1accd386820a985a6dc2fd92dec5b/pcgrad.py#L58): 

```
if self._reduction:
    merged_grad[shared] = torch.stack([g[shared]
                                   for g in pc_grad]).mean(dim=0)
```

because if reduction is a string, self._reduction is always True