patch: CN mirrors for pytorch-wheels and HF datasets

- pyproject.toml + uv.lock: pytorch-cu128/cpu indexes → mirror.sjtu.edu.cn
  (aliyun lacks 2.9.1, sjtu has it)
- nanochat/dataset.py: climbmix BASE_URL → hf-mirror.com

For ailab (CN, RTX 5090) where direct pytorch.org and huggingface.co
are unreachable. Override at uv-sync time with UV_DEFAULT_INDEX env.

pyproject.toml

@@ -44,12 +44,12 @@ torch = [
 
 [[tool.uv.index]]
 name = "pytorch-cpu"
-url = "https://download.pytorch.org/whl/cpu"
+url = "https://mirror.sjtu.edu.cn/pytorch-wheels/cpu"
 explicit = true
 
 [[tool.uv.index]]
 name = "pytorch-cu128"
-url = "https://download.pytorch.org/whl/cu128"
+url = "https://mirror.sjtu.edu.cn/pytorch-wheels/cu128"
 explicit = true
 
 [project.optional-dependencies]