refactor (csrc): Restructure C++ code organization to facilitate adding new kernels

[lcy-seso]

This PR does not change any logic or functionality in the main branch. It restructures the existing C++ code organization to facilitate the addition of new kernels in subsequent PRs.

In general, each customized CUDA kernel involves modifying the following three files at the backend:

1. Registering Python bindings and adding declarations in ops.cc.
2. Placing the host's kernel launch function in x.cc, where x is the operator's name.
3. Implementing CUDA kernels in x.cuh.