Add CUDA backend

[mawad-amd]

Summary

Allow Iris to be installed with --config-settings backend=nvidia so that iris.hip.* imports transparently redirect to CUDA equivalents (iris.cuda.*). This enables NVIDIA backend support without changing source code.

Goal

At install or runtime, users should be able to do:

pip install git+https://github.com/ROCm/iris.git --config-settings backend=nvidia

and have:

from iris.hip.runtime import hip_malloc

automatically resolve to:

from iris.cuda.runtime import hip_malloc

Cherry pick commits from https://github.com/astroC86/iris/tree/astroC86/cuda-port and #118