Add experimental support for cuStateVecEx.

[sergeisakov]

No description provided.

[gemini-code-assist]

Code Review

This pull request adds experimental support for cuStateVecEx, which is a great addition for multi-GPU and multi-node simulations. The changes are extensive, touching build systems, the Python interface, and adding a significant amount of new C++ code.

I've found several critical issues in the build files (Makefile, CMakeLists.txt, BUILD) that will prevent the code from compiling or linking correctly. There's also a compilation error in apps/qsim_base_custatevecex.cu.

Additionally, there are several violations of the C++ style guide regarding C-style casts, which should be replaced with C++-style casts (static_cast, reinterpret_cast). I've pointed out one instance per file and requested that all occurrences be fixed. There's also a code smell with const_cast that could be improved with a better design.

Please address these issues to ensure the new functionality is robust and maintainable. The addition of tests for the new backend is much appreciated.