fix: use --torch-backend instead of --extra-index-url for dependency compilation

The old --extra-index-url approach caused uv to resolve any package from
the PyTorch wheel index when it happened to be available there, not just
torch-ecosystem packages.

Replace it with uv's --torch-backend flag which routes only torch,
torchaudio, torchvision, and torchsde to the correct PyTorch index.
Map NVIDIA to cu126, AMD to rocm6.1, and CPU to the cpu index so each
backend gets the right torch variant.

Pin uv>=0.6.0 in pyproject.toml as the minimum version supporting
--torch-backend.

Author: bigcat88

comfy_cli/uv.py

@@ -64,9 +64,11 @@ def parse_req_file(rf: PathLike, skips: list[str] | None = None):
 
 
 class DependencyCompiler:
+    cpuPytorchUrl = "https://download.pytorch.org/whl/cpu"
     rocmPytorchUrl = "https://download.pytorch.org/whl/rocm6.1"
     nvidiaPytorchUrl = "https://download.pytorch.org/whl/cu126"
 
+    cpuTorchBackend = "cpu"
     rocmTorchBackend = "rocm6.1"
     nvidiaTorchBackend = "cu126"
 
@@ -387,11 +389,13 @@ def __init__(
         self.gpuUrl = (
             DependencyCompiler.nvidiaPytorchUrl if self.gpu == GPU_OPTION.NVIDIA else
             DependencyCompiler.rocmPytorchUrl if self.gpu == GPU_OPTION.AMD else
+            DependencyCompiler.cpuPytorchUrl if self.gpu == GPU_OPTION.CPU else
             None
         )  # fmt: skip
         self.torchBackend = (
             DependencyCompiler.nvidiaTorchBackend if self.gpu == GPU_OPTION.NVIDIA else
             DependencyCompiler.rocmTorchBackend if self.gpu == GPU_OPTION.AMD else
+            DependencyCompiler.cpuTorchBackend if self.gpu == GPU_OPTION.CPU else
             None
         )  # fmt: skip
         self.out: Path = self.outDir / outName

tests/uv/test_uv.py

@@ -94,8 +94,8 @@ def test_torch_backend_amd():
 
 def test_torch_backend_cpu():
     depComp = DependencyCompiler(cwd=temp, gpu=GPU_OPTION.CPU, outDir=temp, reqFilesCore=[], reqFilesExt=[])
-    assert depComp.torchBackend is None
-    assert depComp.gpuUrl is None
+    assert depComp.torchBackend == "cpu"
+    assert depComp.gpuUrl == DependencyCompiler.cpuPytorchUrl
 
 
 def test_torch_backend_none():