Add support for modeling readout errors in noise model

Required prerequisites

• Search the issue tracker to check if your feature has already been mentioned or rejected in other issues.

Describe the feature

Background

Readout errors are caused by imperfect qubit measurement, and are a common source of error in quantum computing. Properly modeling these errors in simulation can give the user tools to better understand how these errors when running on actual quantum devices. There is a large body of work studying readout error, particularly in the context of how to mitigate it [1], [2].

Description

Expand the CUDA-Q NoiseModel (https://github.com/NVIDIA/cuda-quantum/blob/main/runtime/common/NoiseModel.h) to include readout errors.

The implementation should include an independent qubit measurement error model, where only two parameters are needed from the user (the probability that 0 is measured as 1, and the probability that 1 is measured as 0), and applied to each measurement.

The implementation should also include a full multi-qubit error model, where the full confusion matrix is used as input, which describes how each possible bit-string output maps to the erroneous bit-strings. The readout error can be applied by multiplying the noise-free measurements after sample has been called, and the simulation has finished.

This feature requested first in issue #891.

References

[1] https://mitiq.readthedocs.io/en/stable/guide/rem-5-theory.html
[2] https://arxiv.org/pdf/2006.14044

Start with runtime/common/NoiseModel.h and trace how noise-free measurements are produced after sample. Review the linked readout-error references and issue #891 for context. Done means supporting independent per-qubit 0-to-1 and 1-to-0 probabilities plus a full multi-qubit confusion matrix applied to measurement results.