Move predictions to CPU before accumulating

[EricWiener]

What does this PR do?

Moves predictions to CPU before accumulating.

See discussion: #7485

Does your PR introduce any breaking changes? If yes, please list them.

Predictions are stored on cpu instead of device.

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[tchaton]

LGTM !
Note: I believe we shouldn't do this before the on_predict_batch_end as some users might do some post processing on device.

[codecov]

Codecov Report

Merging #9085 (3f7d0ed) into master (6f3cb1a) will decrease coverage by 4%.
The diff coverage is 100%.

❗ Current head 3f7d0ed differs from pull request most recent head e1e242a. Consider uploading reports for the commit e1e242a to get more accurate results

@@           Coverage Diff           @@
##           master   #9085    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         178     178            
  Lines       14692   14693     +1     
=======================================
- Hits        13524   12918   -606     
- Misses       1168    1775   +607     

[carmocca]

LGTM

Missing the torch import