Merge generic device helper functions into gpu set.

[trivialfis]

• Remove the use of old NDevices* functions.

@RAMitchell Some planed changes from #3608 extracted here to let that PR less messy. To complete that PR, some functions need to be generalized for both CPU and GPU. Since I can't test it on multi-gpu environment, I think it's best to split this as an independent PR to ease reviewing.

[trivialfis]

@RAMitchell , @canonizer I think I need to rebase it on #3446 . Otherwise this should be all of it once tests passed. Please help reviewing, I can't test it on multi-gpu environment. Thanks in advance. :)

[codecov-io]

Codecov Report

Merging #3626 into master will increase coverage by 0.23%.
The diff coverage is 97.91%.

@@             Coverage Diff              @@
##             master    #3626      +/-   ##
============================================
+ Coverage     50.39%   50.62%   +0.23%     
  Complexity      188      188              
============================================
  Files           172      174       +2     
  Lines         13986    14031      +45     
  Branches        457      457              
============================================
+ Hits           7048     7103      +55     
+ Misses         6713     6703      -10     
  Partials        225      225

Impacted Files	Coverage Δ	Complexity Δ
src/common/host_device_vector.h	100% <ø> (ø)	0 <0> (ø)	⬇️
src/common/timer.h	56.52% <ø> (ø)	0 <0> (ø)	⬇️
src/common/common.h	100% <100%> (ø)	0 <0> (ø)	⬇️
tests/cpp/common/test_gpuset.cc	100% <100%> (ø)	0 <0> (?)
src/common/gpu_set.h	95.45% <95.45%> (ø)	0 <0> (?)
python-package/xgboost/libpath.py	62.5% <0%> (-8.34%)	0% <0%> (ø)
src/data/sparse_page_dmatrix.cc	94.67% <0%> (+0.59%)	0% <0%> (ø)	⬇️
src/data/sparse_page_raw_format.cc	64.15% <0%> (+22.64%)	0% <0%> (ø)	⬇️

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[RAMitchell]

LGTM