After reading the document of setup script (2. Writing the Setup Script — Python 3.10.1 documentation), i modify the setup.py file (for cuda folder) as follows, and we can import lltm as installed module :
from setuptools import setup
from torch.utils.cpp_extension import BuildExtension, CUDAExtension

setup(
    name='lltm_cuda',
    py_modules = ['lltm'],
    ext_modules=[
        CUDAExtension('lltm_cuda', [
            'lltm_cuda.cpp',
            'lltm_cuda_kernel.cu',
        ]),
    ],
    cmdclass={
        'build_ext': BuildExtension
    })