focal_stats(): gpu case

[thuydotm]

Fixes #707

[codecov-commenter]

Codecov Report

Merging #709 (326b441) into master (dad5abb) will decrease coverage by 1.71%.
The diff coverage is 23.33%.

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
- Coverage   81.58%   79.87%   -1.72%     
==========================================
  Files          19       19              
  Lines        4057     4169     +112     
==========================================
+ Hits         3310     3330      +20     
- Misses        747      839      +92     

Impacted Files	Coverage Δ
xrspatial/focal.py	56.01% <23.33%> (-16.83%)	⬇️

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