make student_model_path optional in distillation

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (8e8bc16) 87.98% compared to head (efcfec5) 87.98%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #173 +/- ## ======================================= Coverage 87.98% 87.98% ======================================= Files 81 81 Lines 4045 4045 ======================================= Hits 3559 3559 Misses 486 486 ``` | [Files](https://app.codecov.io/gh/deepmodeling/dpgen2/pull/173?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling) | Coverage Δ | | |---|---|---| | [dpgen2/entrypoint/args.py](https://app.codecov.io/gh/deepmodeling/dpgen2/pull/173?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBnZW4yL2VudHJ5cG9pbnQvYXJncy5weQ==) | `91.26% <ø> (ø)` | | | [dpgen2/entrypoint/submit.py](https://app.codecov.io/gh/deepmodeling/dpgen2/pull/173?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBnZW4yL2VudHJ5cG9pbnQvc3VibWl0LnB5) | `68.95% <100.00%> (ø)` | |

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