[SYSTEMML-1325] Harmonize Compilation Execution Pipelines and Add GPU Support to JMLC

[thomas9t]

This PR adds support for compilation and execution of GPU enabled scripts in JMLC and harmonizes the pipeline used to compile and execute DML programs across the JMLC, MLContext and DMLScript. Specifically, the following changes were made:

1. All three APIs now call ScriptExecutorUtils.compileRuntimeProgram to compile DML scripts. The original logic in MLContext and JMLC for pinning inputs and persisting outputs has been preserved.
2. All three APIs now use ScriptExecutorUtils.executeRuntimeProgram to execute the compiled program. Previously, JMLC called the Script.execute method directly.
3. jmlc.Connection.prepareScript now supports compiling a script to use GPU. Note that following [SYSTEMML-1325] Cleanup static variables in DMLScript #832 the issue noted in [SYSTEMML-1325] Support for GPU in JMLC and Harmonize Script Execution Pipelines #830 has been resolved.
4. A PreparedScript is now statically assigned a GPU context when it is compiled and instatiated. This has potential performance implications because it means that a PreparedScript must be executed on a specific GPU. However, it reduces overhead from creating a GPU context each time a script is executed and unsures that a user cannot compile a script to use GPU and then forget to assign a GPU context when the script is run.
5. Per (3) I have added a unit test which compiles and executes a GPU enabled script in JMLC both with and without pinned data and just asserts that no errors occur.

I ran mvn clean verify on a GPU enabled machine and encountered no exceptions.

@niketanpansare can you please take a look and let me know what you think? @mboehm7 you may also want to look at this.

[niketanpansare]

Thanks @thomas9t ... the PR looks good to me 👍

Let's wait until tomorrow before merging in case @mboehm7 has additional comments.