Scaling issue with Gramian computation for RowMatrix: too many results sent to driver

This issue hit me when running PCA on large dataset (~1Billion rows, ~30k columns).

Computing Gramian of a big RowMatrix allows to reproduce the issue.

The problem arises in the treeAggregate phase of the gramian matrix computation: results sent to driver are enormous.

A potential solution to this could be to replace the hard coded depth (2) of the tree aggregation by a heuristic computed based on the number of partitions, driver max result size, and memory size of the dense vectors that are being aggregated, cf below for more detail:

(nb_partitions)^(1/depth) * dense_vector_size <= driver_max_result_size

I have a potential fix ready (currently testing it at scale), but I'd like to hear the community opinion about such a fix to know if it's worth investing my time into a clean pull request.

Note that I only faced this issue with spark 2.2 but I suspect it affects later versions aswell.