[AutodiffScalar] Remove const when returning by value

clang-tidy: Return type 'const T' is 'const'-qualified at the top level,
which may reduce code readability without improving const correctness

The types are somewhat long, but the affected return types are of the form:
```
const T my_func() { /**/ }
```

Change to:
```
T my_func() { /**/ }
```