relax type annotation

[YichengDWu]

Closes #37

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (19c10f2) 86.44% compared to head (5f2cbfb) 86.44%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #38 +/- ## ======================================= Coverage 86.44% 86.44% ======================================= Files 5 5 Lines 214 214 ======================================= Hits 185 185 Misses 29 29 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaDiff/TaylorDiff.jl/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaDiff) | Coverage Δ | | |---|---|---| | [src/chainrules.jl](https://app.codecov.io/gh/JuliaDiff/TaylorDiff.jl/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaDiff#diff-c3JjL2NoYWlucnVsZXMuamw=) | `72.30% <100.00%> (ø)` | |

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[tansongchen]

LGTM, thanks