Refactor GGUF loading: new converter chain & GGUFQuantizer
- Replace per-arch TensorProcessor classes with declarative
GGUFDequantizer converters wired into the conversion-ops chain
- Add GGUFQuantizer + GGUFQuantizedTensor for on-the-fly dequant via
the unified loading path in core_model_loading
- Simplify load_gguf_checkpoint signature; drop dummy-model state-dict
pre-mapping and get_gguf_hf_weights_map
- Add pytest-benchmark dep and ggml benchmark test