bugfix: workspace dir when no GPU is available (#579)

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.

python/flashinfer/jit/env.py

@@ -16,13 +16,22 @@
 
 import pathlib
 import re
+import warnings
 
 from torch.utils.cpp_extension import _get_cuda_arch_flags
 
 
 def _get_workspace_dir_name() -> pathlib.Path:
-    flags = _get_cuda_arch_flags()
-    arch = "_".join(sorted(set(re.findall(r"compute_(\d+)", "".join(flags)))))
+    try:
+        with warnings.catch_warnings():
+            # Ignore the warning for TORCH_CUDA_ARCH_LIST not set
+            warnings.filterwarnings(
+                "ignore", r".*TORCH_CUDA_ARCH_LIST.*", module="torch"
+            )
+            flags = _get_cuda_arch_flags()
+        arch = "_".join(sorted(set(re.findall(r"compute_(\d+)", "".join(flags)))))
+    except Exception:
+        arch = "noarch"
     # e.g.: $HOME/.cache/flashinfer/75_80_89_90/
     return pathlib.Path.home() / ".cache" / "flashinfer" / arch