Refactor source code parity check with `ast`

[XuehaiPan]

Use ast.unparse(ast.parse(code)) to normalize the source code. This ignores formatting differences and allows us to enable code formattor on torch upstream.

As per:

• https://github.com/pytorch/pytorch/pull/128594#issuecomment-2181558483
• https://github.com/pytorch/pytorch/pull/128594#discussion_r1649795743