[REVIEW] Moving some linalg and stats prims to RAFT

[divyegala]

This PR moves some primes that I mistakenly left out from moving before, which include:

1. linalg/norm.cuh
2. linalg/reduce.cuh
3. linalg/coalesced_reduction.cuh
4. linalg/strided_reduction.cuh
5. stats/sum.cuh

There are no API changes in this PR, and there is only a switch of namespaces

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov-io]

Codecov Report

Merging #3044 into branch-0.17 will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           branch-0.17    #3044   +/-   ##
============================================
  Coverage        59.26%   59.26%           
============================================
  Files              142      142           
  Lines             8962     8962           
============================================
  Hits              5311     5311           
  Misses            3651     3651           

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