Move compute_column API out of ast namespace

[vyasr]

The compute_column API is functionally analogous to cudf::transform, except that the former expresses the transformation in terms of a cudf expression while the latter accepts string UDFs to be JITted and evaluated. Moving compute_column into transform.hpp means that the only public APIs in the ast namespace are those required to define an expression, so we clearly separate the logic for defining and parsing of expressions from APIs (like conditional joins and compute_column) that evaluate those expressions on device and then use the output in different ways.

Contributes to #8145.

[vyasr]

@harrism you requested a better name for the compute_column API if it's moved outside of the ast namespace here. The closest API currently in cudf is the transform function, which is also extremely generic. Any thoughts on how you'd like to move forward with that?

@jlowe I've updated the JNI bindings, so we should be good to go for now. If the function name changes I'll attempt to propagate that through as well, but I may need a bit of help with that to make sure that I get everything since that will also affect all the Java code using this API (right now the change is just a namespace change when calling the C++ API, which is internal to the JNI).

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[vuule]

Looks good. Not sure if I understand why the changes are made, but PR itself is 👍

[vyasr]

Looks good. Not sure if I understand why the changes are made, but PR itself is 👍

This API just says "compute a new column by performing some operation on the rows of a table", which is essentially the same as cudf::transform. That behavior should dictate where the API lives, not the fact that it happens to represent the operation by an AST expression, otherwise APIs like conditional joins would also live in the cudf::ast namespace. In general, since AST expression evaluation could be used in many places we want to limit the cudf::ast namespace to just the logic for defining expressions (and cudf::ast::detail to internal code for parsing and evaluating those expressions in device code).

[robertmaynard]

CMake changes LGTM

[vyasr]

@gpucibot merge

[trxcllnt]

@gpucibot merge