[Feature Request] Missing optimization of DequantizeLinear ∘ Flatten ∘ QuantizeLinear?

[mcollinswisc]

Discussed in https://github.com/microsoft/onnxruntime/discussions/21167

^{Originally posted by **mcollinswisc** June 25, 2024} It looks like ONNXRuntime will optimize DequantizeLinear ∘ Reshape ∘ QuantizeLinear to only the Reshape, eliminating the quantization/de-quantization, if the scales & zero points are the same. However, an equivalent Flatten is not optimized. Is this likely to be just a missing optimization, or is there some reason the qdq would be preserved in this case? Tested out in: https://gist.github.com/mcollinswisc/d1cd9d13b4e5fbad01c75dca5c9ca576 with ONNXRuntime 1.18.0

[skottmckay]

Should be possible to add to this list given the ONNX spec for Flatten allows 8-bit integers: https://github.com/microsoft/onnxruntime/blob/0f1f3b7705ddc2fe4f371f78a8a8b6a0428a68de/onnxruntime/core/optimizer/qdq_transformer/selectors_actions/qdq_selector_action_transformer.cc#L63-L68