Suppress a warning in unsqueeze (#2637)

I keep getting this warning when building PyTorch:

```
In file included from
/home/hong/wsrc/pytorch/third_party/onnx/onnx/defs/tensor/utils.h:6,
                 from
/home/hong/wsrc/pytorch/third_party/onnx/onnx/defs/tensor/defs.cc:4:
/home/hong/wsrc/pytorch/third_party/onnx/onnx/defs/tensor/defs.cc: In
lambda function:
/home/hong/wsrc/pytorch/third_party/onnx/onnx/defs/tensor/defs.cc:1414:22:
warning: unnecessary parentheses in declaration of �i�
[-Wparentheses]
           for (size_t(i) = 0; i < axes.size(); ++i) {
                      ^
/home/hong/wsrc/pytorch/third_party/onnx/onnx/defs/schema.h:959:12:
note: in definition of macro �ONNX_OPERATOR_SET_SCHEMA_EX�
     return impl.SetName(#name)
\
            ^~~~
/home/hong/wsrc/pytorch/third_party/onnx/onnx/defs/tensor/defs.cc:1369:1:
note: in expansion of macro �ONNX_OPERATOR_SET_SCHEMA�
 ONNX_OPERATOR_SET_SCHEMA(
```

This commit should fix it and modernize the code a bit.

Co-authored-by: Ke Zhang <kezhan@microsoft.com>

onnx/defs/tensor/defs.cc

@@ -1578,13 +1578,13 @@ ONNX_OPERATOR_SET_SCHEMA(
           const auto& input_shape = ctx.getInputType(0)->tensor_type().shape();
           const auto input_ndim = input_shape.dim_size();
           const auto output_ndim = input_ndim + static_cast<int>(axes.size());
-          for (size_t(i) = 0; i < axes.size(); ++i) {
-            if (axes[i] < -output_ndim || axes[i] >= output_ndim) {
+          for (auto& axe : axes) {
+            if (axe < -output_ndim || axe >= output_ndim) {
               fail_shape_inference(
                   "values in 'axes' are beyond the bounds of the computed output shape");
             }
-            if (axes[i] < 0) {
-              axes[i] += output_ndim;
+            if (axe < 0) {
+              axe += output_ndim;
             }
           }