Random forest refactoring

[RAMitchell]

Summary of the changes:

• Remove some unused print functions
• Move validity checks into parameter construction, so parameters are checked by default
• Remove Node_ID_info struct, we can just use a std::pair
• Move builder_base.cuh into builder.cuh
• Remove node.cuh. Use InstanceRange to store this information.
• Builder.train() directly returns a DT::TreeMetaDataNode<DataT, LabelT> object
• computeQuantiles is made into a pure function. Some weird usages of smart pointers removed.
• Unused DataInfo struct removed
• DecisionTree class member variables removed, member functions made into pure functions (static)
• Some unnecessary RandomForest member variables removed, destructor removed
• Some instances of new/delete change to use std containers
• Tests for instance counts moved from python to gtest
• Change indexing type from 32-bit integers to std::size_t
• Test fil predictions against rf predictions, fixes a case where ties in multi-class prediction are broken inconsistently in RF's cpu predictor

[venkywonka]

LGreatTM 👍🏾

[vinaydes]

I think changing each IdxT to size_t is not required and is not good for performance. Especially in the device code. For example n_bins would never be more than few thousands (currently max 1024).
It is probably safe for now to assume that n_rows and n_cols are both less than 2^32 and product of the two (size of the dataset) is size_t (i.e. less than 2^64 on 64-bit platform). So any derived variables for n_rows and n_cols, such as n_sampled_cols, could be treated as 32-bit integers. Anything that is derived from the size of dataset, should be size_t.
If we keep IdxT abstraction throughout the code for integer types (n_rows, n_cols) then we can change it to bigger sizes in future if needed.

[RAMitchell]

I reverted the 32->64 bit changes for now as I was not able to resolve a minor performance difference, and it's not that relevant to this pr.

[vinaydes]

Apart from minor hash related comment, everything looks good to go. Approving.

[RAMitchell]

rerun tests

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (branch-21.10@7706130). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             branch-21.10    #4166   +/-   ##
===============================================
  Coverage                ?   85.97%           
===============================================
  Files                   ?      231           
  Lines                   ?    18502           
  Branches                ?        0           
===============================================
  Hits                    ?    15907           
  Misses                  ?     2595           
  Partials                ?        0           

Flag	Coverage Δ
dask	47.33% <0.00%> (?)
non-dask	78.57% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7706130...06f41dc. Read the comment docs.

[dantegd]

@gpucibot merge