Fix NVTX profiling: restore missing node_compute_range_.Begin() call (#28273)

### Description

Restores the missing `node_compute_range_.Begin()` call in
`KernelScope`'s constructor. The corresponding `End()` in the destructor
was kept, so every kernel execution hits:

```
ORT_ENFORCE(is_begin_called_, "End must be called after calling Begin.");
```

### Motivation and Context

PR #26846 refactored `sequential_executor.cc` into RAII scope classes
but dropped the `node_compute_range_.Begin()` call during the move. Any
build with `--enable_nvtx_profile` fails at runtime on the first kernel
execution.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: tianleiwu <30328909+tianleiwu@users.noreply.github.com>

Author: Copilot

onnxruntime/core/framework/sequential_executor.cc

@@ -364,6 +364,7 @@ class KernelScope {
         forward_range.End();
         backward_range.Begin();
       }
+      node_compute_range_.Begin();
     }
 #endif