Add support for 8-bit Quantization

[clebert]

See:

• Quantization Brainstorming karpathy/llama2.c#277
• 8-bit Quantization karpathy/llama2.c#298
• int8 quantization karpathy/llama2.c#312
• int8 quantization attempt #2 karpathy/llama2.c#364
• Investigate alternative approach for Q4 quantization  ggml-org/llama.cpp#397
• https://arxiv.org/pdf/2101.01321v3.pdf