bugfix: workspace dir when no GPU is available

[abcdabcd987]

When GPU is not available (e.g., CUDA_VISIBLE_DEVICES=-1), _get_cuda_arch_flags() raises IndexError.

/opt/venv/lib/python3.10/site-packages/flashinfer/__init__.py:17: in <module>
    from .activation import (
/opt/venv/lib/python3.10/site-packages/flashinfer/activation.py:21: in <module>
    from .jit import (
/opt/venv/lib/python3.10/site-packages/flashinfer/jit/__init__.py:27: in <module>
    from .activation import (
/opt/venv/lib/python3.10/site-packages/flashinfer/jit/activation.py:19: in <module>
    from .env import FLASHINFER_GEN_SRC_DIR
/opt/venv/lib/python3.10/site-packages/flashinfer/jit/env.py:31: in <module>
    FLASHINFER_WORKSPACE_DIR = _get_workspace_dir_name()
/opt/venv/lib/python3.10/site-packages/flashinfer/jit/env.py:24: in _get_workspace_dir_name
    flags = _get_cuda_arch_flags()
/opt/venv/lib/python3.10/site-packages/torch/utils/cpp_extension.py:1984: in _get_cuda_arch_flags
    arch_list[-1] += '+PTX'
E   IndexError: list index out of range

Although FlashInfer is not useful in this case, we still don't want to crash user program when importing flashinfer. This PR fixes this issue.

Another change is to hide the warning of TORCH_CUDA_ARCH_LIST not set when importing flashinfer. This can be annoying when using AOT wheels.