Minor suggestion for pytorch version requirement of UIO2

Since **nn.LayerNorm()** in **pytorch <2.1.0** [seems](https://pytorch.org/docs/2.0/_modules/torch/nn/modules/normalization.html#LayerNorm) to exist without keyword argument `'bias'`,
following error occurs while loading pretrained ckpt of UIO2 model:
```
TypeError: __init__() got an unexpected keyword argument 'bias'
```

Therefore, I recommend whom are interested in testing the pretrained version of UIO2 to try **pytorch >= 2.1.0**.