[MLAS] Fix rotary interleaved NEON kernel (#26390)
The logic of interleaved NEON kernel is not correct from code review:
1. **Test Code Logic:**
The test code `test_rope.h` allocates the `sin` and `cos` tables based
on the `interleaved` flag:
```c++
size_t table_len = interleaved ? rotary_emb_dim / 2 : rotary_emb_dim;
std::vector<float> sin_data(table_len);
std::vector<float> cos_data(table_len);
```
For the `interleaved = true` case, the test creates `sin` and `cos`
tables of length `rotary_emb_dim / 2`.
2. **AVX2 (fp32) Kernel Logic (`interleaved = true`):**
This kernel loads the `sin`/`cos` data using an index of `i / 2`:
```c++
float32x8_t sin_val = _mm256_loadu_ps(sin_data + i / 2);
float32x8_t cos_val = _mm256_loadu_ps(cos_data + i / 2);
```
This logic expects a `sin`/`cos` table of length `rotary_emb_dim / 2`.
**Conclusion: The AVX2 (fp32) kernel is consistent with the test code.**
3. **NEON (fp16) Kernel Logic (`interleaved = true`):**
This kernel loads the `sin`/`cos` data using an index of `i`:
```c++
// Enters loop with sin_val = MlasLoadFloat16x8(sin + i);
//...
// Inside loop, for next iteration:
sin_val = MlasLoadFloat16x8(sin + i + 16);
```
This logic expects a `sin`/`cos` table of length `rotary_emb_dim`.
**Conclusion: The NEON (fp16) kernel is NOT consistent with the test
code.**
### Regression Test
```
cmake --build build/Linux/Release --config Release --target onnxruntime_mlas_test && ./build/Linux/Release/onnxruntime_mlas_test --gtest_filter=NeonFp16RoPE*
```
Before applying the fix, the test failed:
```
[ FAILED ] NeonFp16RoPE.ShortExecute (13 ms)
onnxruntime/onnxruntime/test/mlas/unittest/test_rope_neon_fp16.cpp:66: Failure
Value of: CloseEnough(output_impl[i].ToFloat(), output_ref[i].ToFloat())
Actual: false
Expected: true
Expected bits: 19491 (16.546875) Actual bits: 56596 (-325) @[16], rotary_emb_dim=24, interleaved=true
```
After applying the fix, test passed.
### Summary
The `RopeKernel_Avx2_fp32_Impl<true>` kernel correctly aligns with the
test code (and the fallback implementation) by expecting a `sin`/`cos`
table of length `rotary_emb_dim / 2`.
The `RopeKernel_Fp16_Impl<true>` (NEON) kernel incorrectly expects a
table of length `rotary_emb_dim`. When run against the provided test,
the NEON kernel will read past the end of the `sin_data` and `cos_data`
vectors.
---------
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
