Use argument out_dir not self.out_dir

[nzw0301]

I'm not sure this is intended, but the currently we cannot specify out_dir from the argument because self.out_dir is always used.

[linshokaku]

We have considered revisions in #735, however, either of the following is required:

• A change in the behavior that is non-obvious to users
• Breaking backwards compatibility
  • We believe there may be implications for users who utilize subclasses inheriting the Writer class.

Therefore, while the implementation can lead to misunderstandings, we concluded that it does not amount to a critical issue.