[MLAS] Fix Lut GEMM Flakiness and Accuracy (#27216)
This PR resolves flakiness and accuracy issues in the
`MatMulNBitsLutGemm` operator.
## Root Cause Analysis
The `MatMulNBitsLutGemm` operator exhibited non-deterministic flakiness
and numerical accuracy issues. This analysis covers the root causes
addressed by the changes.
## Identified Root Causes
### 1. Data Race in
[LutGemmPackQuantBData](https://github.com/microsoft/onnxruntime/blob/cee825d34d533ca325bfd8f8269c86133ae512e6/onnxruntime/core/mlas/lib/qlutgemm.cpp#L166-L295)
- **Issue**: The weight packing loop was parallelized across output
features ($N$). Since T-MAC packs multiple features into a single byte,
concurrent updates to the same byte caused bit-level corruption.
- **Fix**: Serialized the sub-byte accumulation phase of the weight
packing process.
### 2. Thread-Safety in Global Configuration Map
- **Issue**: `tmac_kernel_configs` (a static `std::unordered_map`) was
accessed concurrently. Map insertions or rehashing during initialization
could invalidate references held by other threads.
- **Fix**: Added `std::mutex` protection and modified the parameter
getter to return by value.
### 3. Tiling Dimension Mismatch and Buffer Safety
- **Issue**: The orchestrator used batch size ($M$) for kernel
configuration, while weights are tiled by features ($N$). Additionally,
the kernel lacked clamping for partial tiles, leading to potential
overruns.
- **Fix**: Synchronized tiling logic by using $N$ for initialization,
passing `TotalN` for parameter retrieval, and implementing explicit
clamping and tail-case handling in the AVX2 kernel.
### Verification Results
- `MatMulNBitsLutGemm.Float32_2Bits_Asymmetric_Batch32_256x256` passed
100 consecutive iterations.
- Full MatMul2Bits suite passed all 10 tests with standard **0.15f**
tolerance.
