8-bit quantization MVP

[robertknight]

The issue tracks the work involved in an MVP of 8-bit quantization support. The goal is to be able to convert and run:

• A GPT-2 Large model that has been quantized using quantize_dynamic from the onnxruntime.quantization package. This uses MatMulInteger

• A model that uses ConvInteger

• Support i8 and u8 tensors for operator inputs and outputs (Support u8 and i8 tensors in operator inputs, outputs and model files #345)

• Support storing i8 and u8 tensors in rten model files (Support u8 and i8 tensors in operator inputs, outputs and model files #345)

• Support QuantizeLinear, DequantizeLinear, DynamicQuantizeLinear ops (Implement QuantizeLinear, DequantizeLinear, DynamicQuantizeLinear ops #346)

• Support u8 tensors in Gather operator (Support u8 tensors in Gather operator #349)

• Add a script which documents the process of quantizing an ONNX model using dynamic quantisation

  • Enable support for subgraphs

• Add initial non-optimised MatMulInteger implementation

• Implement an initial matmul kernel for u8 x i8 -> i32

• Modify MatMulInteger to use the optimised kernel

• Implement ConvInteger