The boost library unfortunately specializes `conj` for various types and
assumes the original two-template-parameter version. This changes
restores the second parameter. This also restores ABI compatibility.
The specialization for `std::complex` is because `std::conj` is not
a device function. For custom complex scalar types, users should provide
their own `conj` implementation.
We may consider removing the unnecessary second parameter in the future - but
this will require modifying boost as well.
Fixes#2112.
c0eb5f89a4