add __init__ for setup

[wangming1993]

Add init file to folder, that can use python setup tool distributed.
For example, I can call core.util in my python code, when I build E2FGVI as a module

import setuptools

setuptools.setup(
    name='e2fgvi',
    version='1.0',
    description='An End-to-End Framework for Flow-Guided Video Inpainting',
    url='https://github.com/MCG-NKU/E2FGVI', 
    packages = setuptools.find_packages('./')
)

So that I can use like that:

from E2FGVI.core.utils import to_tensors

[wangming1993]

@Paper99 pls take a look, thx