bugfix: fix cli error when cuda toolkit is not installed (#1905)

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## 📌 Description

This is a bugfix to #1904.

## 🔍 Related Issues

#1904 

## 🚀 Pull Request Checklist

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request, please make sure the following items are complete.

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(or used your preferred method).
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- [ ] I have run the hooks manually with `pre-commit run --all-files`
and fixed any reported issues.

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## 🧪 Tests

- [ ] Tests have been added or updated as needed.
- [ ] All tests are passing (`unittest`, etc.).

## Reviewer Notes

cc @bbartels

Author: yzh119

flashinfer/__main__.py

@@ -15,6 +15,7 @@
 """
 
 # flashinfer-cli
+import os
 import click
 from tabulate import tabulate  # type: ignore[import-untyped]
 
@@ -79,13 +80,14 @@ def cli(ctx, download_cubin_flag):
     "FLASHINFER_CUDA_ARCH_LIST": current_compilation_context.TARGET_CUDA_ARCHS,
     "FLASHINFER_CUDA_VERSION": get_cuda_version(),
     "FLASHINFER_CUBINS_REPOSITORY": FLASHINFER_CUBINS_REPOSITORY,
-    "CUDA_HOME": get_cuda_path(),
     "CUDA_VERSION": get_cuda_version(),
 }
 try:
     env_variables["CUDA_HOME"] = get_cuda_path()
+    found_nvcc = os.path.isfile(os.path.join(env_variables["CUDA_HOME"], "bin", "nvcc"))
 except Exception:
-    env_variables["CUDA_HOME"] = "Not Found"
+    env_variables["CUDA_HOME"] = ""
+    found_nvcc = False
 
 
 @cli.command("show-config")
@@ -124,13 +126,23 @@ def show_config_cmd():
     click.secho("=== Torch Version Info ===", fg="yellow")
     click.secho("Torch version:", fg="magenta", nl=False)
     click.secho(f" {torch.__version__}", fg="cyan")
+    click.secho("CUDA runtime available:", fg="magenta", nl=False)
+    if torch.cuda.is_available():
+        click.secho(" Yes", fg="green")
+    else:
+        click.secho(" No", fg="red")
     click.secho("", fg="white")
 
     # Section: Environment Variables
     click.secho("=== Environment Variables ===", fg="yellow")
     for name, value in env_variables.items():
         click.secho(f"{name}:", fg="magenta", nl=False)
         click.secho(f" {value}", fg="cyan")
+    click.secho("NVCC found:", fg="magenta", nl=False)
+    if found_nvcc:
+        click.secho(" Yes", fg="green")
+    else:
+        click.secho(" No", fg="red")
     click.secho("", fg="white")
 
     # Section: Artifact path