Make torch an extra/dev dependency (#50)

To support use of this package when Torch is optional.

.github/workflows/test.yml

@@ -52,3 +52,8 @@ jobs:
 
       - name: Run tests
         run: uv run pytest tests
+
+      - name: Import check without torch
+        run: |
+          uv pip uninstall torch
+          python -c "import kernels"

pyproject.toml

@@ -14,7 +14,6 @@ dependencies = [
   "huggingface-hub>=0.26.3",
   "packaging>=24.2",
   "tomli>=2.0.1; python_version<'3.11'",
-  "torch>=2.5",
 ]
 
 [build-system]
@@ -27,8 +26,12 @@ dev = [
   "pytest >=8",
   # Whatever version is compatible with pytest.
   "pytest-benchmark",
+  "torch >=2.5",
 ]
 
+[project.optional-dependencies]
+torch = ["torch"]
+
 [project.scripts]
 kernels = "kernels.cli:main"