rename `Scalar` to `Scalarlayer` and add `Scalar` as a constant function

[codecov[bot]]

Codecov Report

Base: 29.41% // Head: 29.24% // Decreases project coverage by -0.17% :warning:

Coverage data is based on head (635f4c4) compared to base (0afdea7). Patch coverage: 66.66% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #130 +/- ## ========================================== - Coverage 29.41% 29.24% -0.18% ========================================== Files 16 16 Lines 860 865 +5 ========================================== Hits 253 253 - Misses 607 612 +5 ``` | [Impacted Files](https://codecov.io/gh/YichengDWu/Sophon.jl/pull/130?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Shambles) | Coverage Δ | | |---|---|---| | [src/compact/NeuralPDE/pinn\_types.jl](https://codecov.io/gh/YichengDWu/Sophon.jl/pull/130/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Shambles#diff-c3JjL2NvbXBhY3QvTmV1cmFsUERFL3Bpbm5fdHlwZXMuamw=) | `87.17% <ø> (ø)` | | | [src/layers/operators.jl](https://codecov.io/gh/YichengDWu/Sophon.jl/pull/130/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Shambles#diff-c3JjL2xheWVycy9vcGVyYXRvcnMuamw=) | `100.00% <ø> (ø)` | | | [src/layers/basic.jl](https://codecov.io/gh/YichengDWu/Sophon.jl/pull/130/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Shambles#diff-c3JjL2xheWVycy9iYXNpYy5qbA==) | `74.52% <57.14%> (-3.69%)` | :arrow_down: | | [src/layers/nets.jl](https://codecov.io/gh/YichengDWu/Sophon.jl/pull/130/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Shambles#diff-c3JjL2xheWVycy9uZXRzLmps) | `98.75% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Shambles). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Shambles)

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