## What changes were proposed in this pull request?
* changes the implementation of gemv with transposed SparseMatrix and SparseVector both in mllib-local and mllib (identical)
* adds a test that was failing before this change, but succeeds with these changes.
The problem in the previous implementation was that it only increments `i`, that is enumerating the columns of a row in the SparseMatrix, when the row-index of the vector matches the column-index of the SparseMatrix. In cases where a particular row of the SparseMatrix has non-zero values at column-indices lower than corresponding non-zero row-indices of the SparseVector, the non-zero values of the SparseVector are enumerated without ever matching the column-index at index `i` and the remaining column-indices i+1,...,indEnd-1 are never attempted. The test cases in this PR illustrate this issue.
## How was this patch tested?
I have run the specific `gemv` tests in both mllib-local and mllib. I am currently still running `./dev/run-tests`.
## ___
As per instructions, I hereby state that this is my original work and that I license the work to the project (Apache Spark) under the project's open source license.
Mentioning dbtsai, viirya and brkyvz whom I can see have worked/authored on these parts before.
Author: Bjarne Fruergaard <bwahlgreen@gmail.com>
Closes#15296 from bwahlgreen/bugfix-spark-17721.
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