Enable 2bit CPU matmul fallback (#25582)
### Description
- enable 2bit matmulnbits
- falls back to ComputeBUnpacked (dequants to fp32)
- Also adapting quantize script to enable 2 bits
- adds 2bit unit tests
- [blockwise quantize for 2bits already implemented
](https://github.com/microsoft/onnxruntime/blob/b9575476e94daa9c6578aba92d8f04324dd15815/onnxruntime/core/mlas/lib/q4_dq.cpp#L407)
### Motivation and Context
- working on enabling bitnet + lowbit LLM's
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Co-authored-by: Hector Li <hecli@microsoft.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
