LowerToBackendContract: Explicitly error out on unimplemented operator

lib/Dialect/Torch/Transforms/LowerToBackendContract.cpp

@@ -197,6 +197,17 @@ static bool satisfiesBackendContract(ModuleOp module,
   if (walkResult0.wasInterrupted())
     return false;
 
+  // Check for unimplemented operators first to give more direct diagnostics.
+  walkResult0 = module.walk([&](Torch::OperatorOp op) {
+    if (actuallyEmitDiagnostics) {
+      op->emitError("unsupported by backend contract: Unimplemented operator '"
+        + op.getName() + "'");
+    }
+    return WalkResult::interrupt();
+  });
+  if (walkResult0.wasInterrupted())
+    return false;
+
   // Check all the types of all Value's in the program and the legality of all
   // the ops.
   //

test/Dialect/Torch/verify-backend-contract-unimplemented-op.mlir

@@ -0,0 +1,10 @@
+// RUN: torch-mlir-opt -torch-verify-backend-contract-no-decompositions -split-input-file -verify-diagnostics %s
+func.func @forward(%arg0: !torch.vtensor<[3,5],f32>) -> !torch.vtensor {
+  %none = torch.constant.none
+  %0 = torch.tensor_static_info_cast %arg0 : !torch.vtensor<[3,5],f32> to !torch.vtensor<*,f32>
+  %1 = torch.copy.to_tensor %0 : !torch.tensor<*,f32>
+  // expected-error @+1 {{unsupported by backend contract: Unimplemented operator 'an.unimplemented.op'}}
+  %2 = torch.operator "an.unimplemented.op"(%1, %1, %none) : (!torch.tensor<*,f32>, !torch.tensor<*,f32>, !torch.none) -> !torch.tensor
+  %3 = torch.copy.to_vtensor %2 : !torch.vtensor
+  return %3 : !torch.vtensor
+}