support auto fallback to cpu kernel for cusom device (#44639)

paddle/phi/core/kernel_factory.cc

@@ -110,6 +110,7 @@ const Kernel& KernelFactory::SelectKernelOrThrowError(
                  << "] is not registered.";
   }
 #endif
+
   auto kernel_iter = iter->second.find(kernel_key);
   // TODO(chenweihang): polish refind impl here
   if (kernel_iter == iter->second.end() &&
@@ -118,6 +119,22 @@ const Kernel& KernelFactory::SelectKernelOrThrowError(
         kernel_key.backend(), phi::DataLayout::ALL_LAYOUT, kernel_key.dtype());
     kernel_iter = iter->second.find(any_layout_kernel_key);
   }
+
+#ifdef PADDLE_WITH_CUSTOM_DEVICE
+  if (kernel_iter == iter->second.end()) {
+    // Fallback CPU backend
+    phi::KernelKey cpu_kernel_key(
+        phi::Backend::CPU, kernel_key.layout(), kernel_key.dtype());
+    kernel_iter = iter->second.find(cpu_kernel_key);
+    if (kernel_iter == iter->second.end() &&
+        kernel_key.layout() != phi::DataLayout::ALL_LAYOUT) {
+      phi::KernelKey any_layout_kernel_key(
+          phi::Backend::CPU, phi::DataLayout::ALL_LAYOUT, kernel_key.dtype());
+      kernel_iter = iter->second.find(any_layout_kernel_key);
+    }
+  }
+#endif
+
   PADDLE_ENFORCE_NE(
       kernel_iter,
       iter->second.end(),