[REVIEW] Add dictionary support to cudf::reduce

[davidwendt]

Reference #5963

This PR adds dictionary column type support to the set of cudf::reduce functions.
This PR depends on utilities added in PR #6651

Here are the reduce operations that will be included in this PR.

• all
• any
• max
• mean
• median
• min
• nth_element
• product
• quantile
• std
• sum_of_squares
• sum
• unique_count
• var

[codecov]

Codecov Report

Merging #6666 (95f02a7) into branch-0.17 (1771a8f) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           branch-0.17    #6666   +/-   ##
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  Coverage        81.94%   81.94%           
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  Files               96       96           
  Lines            16164    16164           
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  Hits             13246    13246           
  Misses            2918     2918           

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[davidwendt]

Some of cudf::reduce operations uses cudf::quantile which does not support dictionary columns.
So, I'll have to update cudf::quantile before finishing this PR.

[davidwendt]

Ok, the last few builds have failed because they timed out (>40 minutes!). Adding dictionary to the simple reduce functions (min, max, sum, sum-of-squares, and product) increase their compile time from ~5 mins to ~18 mins. For now, I've included changes to remove the 2nd type-dispatch call. And the compile time for these files are now ~50 seconds. All tests are passing but benchmarks have not been checked. I will create an issue to discuss these changes and then will likely redo them in a separate PR.

[davidwendt]

Created PR #6727 to fix the compile-time issues. Now this PR will become dependent on that one.

[karthikeyann]

Overall LGTM.
Few minor changes.