[Feature Request] Fp6 datatype support

### Describe the feature request

Allow for Onnxruntime to parse and support the FP6 datatype for Onnx models.

Currently in the Onnx spec this doesn't exist yet but I've gotten a feature request started on ONNX as well

https://github.com/onnx/onnx/issues/7048

Lookng to leverage this within MIGraphX as well as the MIGraphX Execution Provider for prequantized models

### Describe scenario use case

Having execution providers take in fp6 (any bit scheme) quantized models and support inference

Start by reviewing the linked ONNX issue 7048 and the existing MIGraphX and MIGraphX Execution Provider integration points. Determine how an FP6 datatype should be represented while the ONNX specification is still missing it, including the requested bit schemes. Done means ONNX Runtime execution providers can accept prequantized FP6 ONNX models and run inference.

Written by the indexing model from the issue text.