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