Purpose
I find Taichi users have a demand for AMDGPU. (related issue #4586)
There is also a discussion of the AMD GPU backend here #412
Thus, I would like to add an AMDGPU backend to the compiler so that my 6900XT can be utilized.

Solution
The purpose is to give Taichi preliminary support for AMDPU via LLVM-14 and ROCm.
I started by completing the following steps step by step

1. the relevant dependencies in cmake
2. a specific version of Taichi-runtime module(taichi/runtime/llvm/runtime_module) for AMDGPU. During the implementation, I found that I couldn't reuse the CPU's llvm directly like taichi does for CUDA. I needed to do some addrspace conversions, so I temporarily used hipcc to help me deal with these issues. (The final approach taken is to hack address space, so we don't need an extra runtime.cpp)
3. interaction between Taichi and ROCm library functions. In cuda backend, some driver functions are used. However, It's convenient to replace these with functions provided by ROCm, such as hipMemcpy, hipModuleLaunchKernel.
4. a codegen skeleton (like taichi/codegen/cuda)
5. convert llvm IR to amdgpu ISA / hsaco(ref link)
6. Unit test configuration for amdgpu.