[Feature] Request to update install-cn.ps1 script to support downloading torch==2.7.0+cu128 and torchvision==0.22.0+cu128 from a mirror source

In the install-cn.ps1 script, line 41 currently uses:
```
python -m pip install torch==2.7.0+cu128 torchvision==0.22.0+cu128 --index-url https://download.pytorch.org/whl/cu128
```
I propose changing it to:
```
python -m pip install torch==2.7.0+cu128 torchvision==0.22.0+cu128 --index-url https://mirrors.nju.edu.cn/pytorch/whl/cu128
```
I have personally tested this change and confirmed that it can correctly and quickly pull the corresponding torch versions. This should significantly improve installation speed for users in China