gen-flake-outputs: add backendBundle output

lib/gen-flake-outputs.nix

@@ -40,6 +40,21 @@ let
 
   applicableBuildSets = build.applicableBuildSets { inherit path buildSets; };
 
+  buildConfigBackend =
+    buildConfig:
+    if buildConfig.cpu or false then
+      "cpu"
+    else if buildConfig ? cudaVersion then
+      "cuda"
+    else if buildConfig ? rocmVersion then
+      "rocm"
+    else if buildConfig ? xpuVersion then
+      "xpu"
+    else if buildConfig.metal or false then
+      "metal"
+    else
+      throw "Cannot determine framework for build set";
+
   # For picking a default shell, etc. we want to use the following logic:
   #
   # - Prefer bundle builds over non-bundle builds.
@@ -60,19 +75,7 @@ let
 
         buildConfig // {
           bundleBuild = buildConfig.bundleBuild or false;
-          framework =
-            if buildConfig.cpu or false then
-              "cpu"
-            else if buildConfig ? cudaVersion then
-              "cuda"
-            else if buildConfig ? rocmVersion then
-              "rocm"
-            else if buildConfig ? xpuVersion then
-              "xpu"
-            else if buildConfig.metal or false then
-              "metal"
-            else
-              throw "Cannot determine framework for build set";
+          framework = buildConfigBackend buildConfig;
           frameworkOrder = if buildConfig ? cudaVersion then 0 else 1;
           frameworkVersion =
             buildConfig.cudaVersion or buildConfig.rocmVersion or buildConfig.xpuVersion or "0.0";
@@ -138,6 +141,24 @@ in
     {
       inherit bundle;
 
+      # Bundles by backend.
+      backendBundle =
+        let
+          backends = lib.unique (map (set: buildConfigBackend set.buildConfig) applicableBuildSets);
+        in
+        builtins.listToAttrs (
+          builtins.map (backend: {
+            name = backend;
+            value = build.mkTorchExtensionBundle {
+              inherit path doGetKernelCheck;
+              buildSets = builtins.filter (
+                set: buildConfigBackend set.buildConfig == backend
+              ) applicableBuildSets;
+              rev = revUnderscored;
+            };
+          }) backends
+        );
+
       default = bundle;
 
       build-and-copy = writeScriptBin "build-and-copy" ''