[Code scan] relax cannot be imported as a package module

Source: Codex global repository scan of deepmodeling/dpti at commit b719828e7eeb571bd26411430197cd74ea78e38c.
Project: https://github.com/orgs/deepmodeling/projects/27

Problem
`dpti.relax` imports `lib.*` instead of package-qualified `dpti.lib.*`. This means `import dpti.relax` and `python -m dpti.relax` fail from a normal installed/package context with `ModuleNotFoundError: No module named 'lib'`.

Code references
https://github.com/deepmodeling/dpti/blob/b719828e7eeb571bd26411430197cd74ea78e38c/dpti/relax.py#L6
https://github.com/deepmodeling/dpti/blob/b719828e7eeb571bd26411430197cd74ea78e38c/dpti/relax.py#L7
https://github.com/deepmodeling/dpti/blob/b719828e7eeb571bd26411430197cd74ea78e38c/dpti/relax.py#L8

Reproduction
Run from the repository root or an installed environment:

```bash
python -c "import dpti.relax"
```

Expected result
Package modules should use package-qualified imports and import cleanly outside direct script execution from inside `dpti/`.