Revert "Use CPU input for test_boost_from_prediction. (#6818)"

[trivialfis]

This reverts commit 74f3a2f.

[trivialfis]

@hcho3 Can we merge this one, and upgrade to 0.19 after its release? (instead of nightly)

[codecov-commenter]

Codecov Report

Merging #6858 (db462ec) into master (b9a4f33) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6858      +/-   ##
==========================================
+ Coverage   81.94%   81.98%   +0.03%     
==========================================
  Files          13       13              
  Lines        3889     3902      +13     
==========================================
+ Hits         3187     3199      +12     
- Misses        702      703       +1     

Impacted Files	Coverage Δ
python-package/xgboost/compat.py	72.38% <0.00%> (-0.17%)	⬇️
python-package/xgboost/core.py	83.19% <0.00%> (ø)
python-package/xgboost/dask.py	81.89% <0.00%> (+0.02%)	⬆️
python-package/xgboost/callback.py	81.86% <0.00%> (+0.04%)	⬆️
python-package/xgboost/sklearn.py	90.52% <0.00%> (+0.16%)	⬆️

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