Fix out of bounds read issue in cross.entropy.cc (#27568)
### Description
Add bounds checking for label tensor values in
`SparseSoftmaxCrossEntropy::Compute` to prevent out-of-bounds memory
reads.
The `SparseSoftmaxCrossEntropy` operator uses `label_data[i]` (int64_t)
directly as an array index into the log-probability buffer without
validating that the value falls within `[0, D)` where `D` is the number
of classes. A malicious ONNX model can embed arbitrary label values in a
model initializer, causing the operator to read heap memory beyond the
log-probability buffer.
Affected expressions in `cross_entropy.cc`:
```cpp
loss_sample[i] = -log_prob_data[i * d + label_data[i]] * weight_data[i]; // weighted path
loss_sample[i] = -log_prob_data[i * d + label_data[i]]; // unweighted path
```
Existing shape validation confirms label and logit dimensions are
compatible, but never validates label **values** against the class
dimension.
## Fix
Added a validation loop before the loss computation that returns an
error status if any label value is outside `[0, D)`:
```cpp
for (ptrdiff_t i = 0; i < n; i++) {
ORT_RETURN_IF(label_data[i] < 0 || label_data[i] >= d,
"SparseSoftmaxCrossEntropy: label value ", label_data[i],
" at index ", i, " is out of range [0, ", d, ")");
}
```
