Add use_existing_torch.py for user who don't want to reinstall torch

[JiamingMai]

For supporting NVIDIA 50 series GPU which uses CUDA 12.8, we can't reinstall torch. This PR adds use_existing_torch.py for user who don't want to reinstall torch. To install torchac_cuda without reinstalling torch, run the following commands:

git clone https://github.com/LMCache/torchac_cuda.git
cd torchac_cuda
python use_existing_torch.py
pip install -r requirements.txt
pip install -e . --no-build-isolation