[REVIEW] RF: code re-organization to enhance build parallelism

[venkywonka]

This PR separates the Decision tree kernels into separate Translation Units (TU) and explicitly instantiates templates.
This is helpful in 2 ways:

1. refactoring top-level RF/DT code now would not require recompilation of the kernels
2. Since they are separated into different TUs and linked, they can leverage build parallelism (4x improvement in rebuild times after touching kernel definitions)

Rebuilding by running time ./build.sh libcuml -v -n PARALLEL_LEVEL=20 after touching RF kernels comparison:
(Note: using --ccache doesn't matter here, assuming after touching RF kernels the state of the code-base is completely new and not part of ccache's hashed index)

This PR

real    0m20.054s             
user    2m28.436s                                               
sys     0m14.241s 

branch-21.12

real    1m21.197s                                                                                                                    
user    2m5.751s                                                                                                                     
sys     0m6.050s

Some other changes include renaming and reorganizing files, pruning headers and cleaning up some code

Things to do:

• split DT Kernels
• benchmark for regressions

[RAMitchell]

I don't like adding a bunch of files but against long compile times it seems like the lesser of two evils.

I think this should go ahead and we can look at other options later.

[venkywonka]

No regressions found on GBM-Bench. The algorithmic correctness, splits generated every iteration, and output tree are the same.
One thing to note is that, due to the code reorganization, the generated kernel code for the computeSplitKernel uses lesser registers than before boosting the occupancy in the GPU, but this doesn't translate to a significant boost/regression in performance.

The below is an average of 100 runs end-to-end times

[venkywonka]

rerun tests

[codecov-commenter]

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[dantegd]

@gpucibot merge