Add typing to accelerators/gpu.py

[carmocca]

What does this PR do?

Part of #7037
Part of #11205

I'm going to try and always have one typing PR open for reviews :)

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

None

Before submitting

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• [n/a] Did you make sure to update the documentation with your changes? (if necessary)
• [n/a] Did you write any new necessary tests? (not for typos and docs)
• [n/a] Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
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PR review

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cc @Borda

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #11333 (71ca214) into master (9771040) will increase coverage by 0%.
The diff coverage is 85%.

❗ Current head 71ca214 differs from pull request most recent head 66379d0. Consider uploading reports for the commit 66379d0 to get more accurate results

@@           Coverage Diff            @@
##           master   #11333    +/-   ##
========================================
  Coverage      92%      92%            
========================================
  Files         194      194            
  Lines       17001    17710   +709     
========================================
+ Hits        15623    16299   +676     
- Misses       1378     1411    +33     

[tchaton]

LGTM !

[akihironitta]

Love the new annotations!