[ONNX][MLIR] add Identity op support (#2754)

Author: PhaneeshB

lib/Conversion/TorchOnnxToTorch/DefaultDomainGtoP.cpp

@@ -560,4 +560,17 @@ void mlir::torch::onnx_c::populateDefaultDomainGtoP(
                     binder.op, resultType, lhs, rhs);
                   return success();
                 });
+  patterns.onOp(
+      "Identity", 14, [](OpBinder binder, ConversionPatternRewriter &rewriter) {
+        Torch::ValueTensorType resultType;
+        Value tensor;
+        if (binder.tensorOperand(tensor) ||
+            binder.tensorResultType(resultType)) {
+          return failure();
+        }
+        Value noneVal = rewriter.create<Torch::ConstantNoneOp>(binder.getLoc());
+        rewriter.replaceOpWithNewOp<Torch::AtenCloneOp>(
+            binder.op, resultType, tensor, /*memory_format=*/noneVal);
+        return success();
+      });
 }

test/Conversion/TorchOnnxToTorch/simple_ops_g_to_p.mlir

@@ -456,3 +456,11 @@ func.func @test_not_2d(%arg0: !torch.vtensor<[3,4],i1>) -> !torch.vtensor<[3,4],
     %0 = torch.operator "onnx.Or"(%arg0, %arg1) : (!torch.vtensor<[3,4],i1>, !torch.vtensor<[3,4],i1>) -> !torch.vtensor<[3,4],i1>
     return %0 : !torch.vtensor<[3,4],i1>
   }
+
+// CHECK-LABEL: func.func @test_identity
+  func.func @test_identity(%arg0: !torch.vtensor<[3,4], f32>) -> !torch.vtensor<[3,4], f32> attributes {torch.onnx_meta.ir_version = 14 : si64, torch.onnx_meta.opset_version = 14 : si64, torch.onnx_meta.producer_name = "backend-test", torch.onnx_meta.producer_version = ""} {
+    // CHECK: %[[NONE:.*]] = torch.constant.none
+    // CHECK: %0 = torch.aten.clone %arg0, %[[NONE]] : !torch.vtensor<[3,4],f32>, !torch.none -> !torch.vtensor<[3,4],f32>
+    %0 = torch.operator "onnx.Identity"(%arg0) : (!torch.vtensor<[3,4], f32>) -> !torch.vtensor<[3,4], f32>
+    return %0 : !torch.vtensor<[3,4], f32>
+  }