Fix the bug about torch.utils.cpp_extension.load

[clownrat6]

When the user doesn't have root right, torch.utils.cpp_extension.load will return error message if we don't assign accessible path to arg:build_directory.

• Set accessible path to arg:build_directory of torch.utils.cpp_extension.load();

• Add Pytorch-0.4.1 with cuda support installation documentation;

• Add .gitignore to ignore some uncessary file;