Releasing `qsim_mgpu` source on GitHub instead of only binaries the Docker container

Dear cuQuantum developers,

After long struggles in trying to get qsim to compile with multi-GPU support, I found out that one cannot accomplish this using stock qsim. IMO this wasn't very clear from the documentation.

Ultimately, I found out by running the NVIDIA cuQuantum Appliance Docker container and checking the git diff --no-index between the original qsim_simulator.py and the ~/conda/envs/cuquantum-23.10/lib/python3.10/site-packages/qsimcirq/qsim_circuit.py file in the Docker container.

It turns out that this changed version uses a module qsim_mgpu which appears to be unavailable outside the said Docker container.

Unfortunately, my current software stack cannot integrate Docker containers (we need full control over the entire environment), yet the multi-GPU support offered by qsim_mgpu is essential for our work. We would greatly benefit from the ability to compile qsim_mgpu independently.

Could you please consider releasing the modified qsim code/fork with qsim_mgpu for standalone use? This would be immensely beneficial for us and potentially for others in the community facing similar challenges.

If there are reasons for keeping this code exclusive to the Docker environment, understanding them could help us explore alternative solutions.

Tagging the core maintainers: @leofang @ahehn-nv @mtjrider @Takuma-Yamaguchi