Missing dtype support for OnnxToTorch

[jinchen62]

Current support: https://github.com/llvm/torch-mlir/blob/main/lib/Conversion/TorchOnnxToTorch/Utils.cpp#L64

According to onnx dtype and torch dtype, missing support:

• 4: onnx.TensorProto.UINT16
• 8: onnx.TensorProto.STRING
• 12: onnx.TensorProto.UINT32
• 13: onnx.TensorProto.UINT64
• 17: onnx.TensorProto.FLOAT8E4M3FN
• 18: onnx.TensorProto.FLOAT8E4M3FNUZ
• 19: onnx.TensorProto.FLOAT8E5M2
• 20: onnx.TensorProto.FLOAT8E5M2FNUZ
• 21: onnx.TensorProto.UINT4
• 22: onnx.TensorProto.INT4

[jinchen62]

https://github.com/pytorch/pytorch/blob/main/c10/core/ScalarType.h#L55

[jinchen62]

https://github.com/shouxieai/tensorRT_Pro/blob/main/onnx/onnx-ml.proto#L487

[jinchen62]

checkpoint
https://github.com/jinchen62/torch-mlir/tree/dtype_support
https://github.com/jinchen62/llvm-project/tree/dtype_support

added float8 types to the op def, it could generates %78 = "llvm.fptrunc"(%76) : (vector<4xf32>) -> vector<4xf8E5M2>, but it fails on https://github.com/llvm/llvm-project/blob/main/mlir/lib/Target/LLVMIR/TypeToLLVM.cpp#L79, and seems there is no support for float8 here https://github.com/llvm/llvm-project/blob/main/llvm/lib/IR/Type.cpp#L234