[rocThrust] use fetch_content to replace download_project

[cenxuantian]

Motivation

rocPRIM has been updated to be using FetchContent vs. DownloadProject due to some of it's shortcomings. To have less variants of the build definition flying around, homogenize them to ease maintenance.

Technical Details

Removed DownloadProjectand replaced it with FetchContent`.

Test Plan

The rocThrust build CI should set everything to default and let rocTrhust to find and download dependencies automatically, in order to do this, we can set FORCE_DEPENDENCIES_DOWNLOAD to ON

Test Result

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1578      +/-   ##
===========================================
+ Coverage    78.35%   83.46%   +5.11%     
===========================================
  Files           24      502     +478     
  Lines         5340    43180   +37840     
  Branches       730     5459    +4729     
===========================================
+ Hits          4184    36040   +31856     
- Misses         943     2898    +1955     
- Partials       213     4242    +4029     

Flag	Coverage Δ
hipSPARSELt	?
rocThrust	83.41% <ø> (?)

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see 526 files with indirect coverage changes

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[umfranzw]

Thanks for doing this. I just had one minor comment.