fix best score on wrong device in EarlyStopping callback

[awaelchli]

What does this PR do?

Fixes #8267

The problematic conversion was introduced in #7959 to fix an issue on TPU, but it can happen now that the current score and the best score are on different devices.

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[codecov]

Codecov Report

Merging #8295 (358e4d3) into master (7ddcdb2) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8295   +/-   ##
======================================
- Coverage      92%     92%   -0%     
======================================
  Files         213     213           
  Lines       13804   13804           
======================================
- Hits        12749   12736   -13     
- Misses       1055    1068   +13     

[tchaton]

LGTM !

[Borda]

chnagelog?

[awaelchli]

@Borda the chnagelog was already added in #7959. This here is just a slight improvement of that same fix.