[BugFix] Pylance emits the warnning 'Code is unreachable'

[lightzhan-intellif]

For the users who use vscode to write code, Pylance in vscode will shadow the code below the function calls tvm.ir.load_json/save_json and tell users "Code is unreachable" like the picture below:

The issue maybe confuses users and make the code a little "dirty".

This PR fixes this by adding a return type hint to the functions load_json and save_json.

[tvm-bot]

Thanks for contributing to TVM! Please refer to the contributing guidelines https://tvm.apache.org/docs/contribute/ for useful information and tips. Please request code reviews from Reviewers by @-ing them in a comment.

• No users to tag found in teams: bugfix _{See #10317 for details}

_{Generated by tvm-bot}