onnxruntime

QuantizeBFP and DequantizeBFP (#12833)

* `QuantizeBFP` and `DequantizeBFP` schemas - similar to
`QuantizeLinear` and `DeQuantizeLinear`.
* BFP datatype is represented as a `uint8` tensor with shape and stride
metadata. This is preferrable to adding a new datatype for BFP, which is
more disruptive and [discouraged by
PyTorch](https://discuss.pytorch.org/t/training-with-custom-quantized-datatype/152132/2).

Context: 

The Microsoft Floating Point (BFP) datatype shares an exponent for every
n numbers called a “bounding box.” Each number still has its own
mantissa and sign bits. BFP has been shown to incur 3-4 less cost
(energy and area) than BFloat16 and INT8 counterparts without reductions
in accuracy for the ImageNet benchmark as described in [Rouhani
2020](https://proceedings.neurips.cc/paper/2020/file/747e32ab0fea7fbd2ad9ec03daa3f840-Paper.pdf).

Requirements:

* There are many variants of BFP (number of mantissa bits, number of
shared exponent bits, size of bounding box, custom bit fields, etc.)
* The size and layout of an BFP variant varies across hardware
* bounding box can be over arbitrary dimensions; for example, for the
channel "C" dimension in a N x C x H x W tensor for convolution

Goals of this PR:

* Add initial versions of QuantizeBFP and DequantizeBFP operators to
enable QDQ-style quantization with BFP. Once the schemas stabilize, we
can consider upstreaming to ONNX.
* Add some basic type and shape inferencing tests; tests that run on an
EP will be a follow-up.