Un-skip some Horovod tests

[carmocca]

What does this PR do?

Fixed by #8116 and/or #6208

Part of #6935

tests/models/test_horovod.py::test_horovod_multi_optimizer PASSED        [ 48%]
tests/models/test_horovod.py::test_horovod_multi_optimizer_with_scheduling_stepping PASSED [ 48%]

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

None

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• [n/a] Did you write any new necessary tests? (not for typos and docs)
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[codecov]

Codecov Report

Merging #8676 (8d75b59) into master (7a1e972) will increase coverage by 4%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #8676    +/-   ##
=======================================
+ Coverage      88%     93%    +4%     
=======================================
  Files         168     168            
  Lines       13984   13985     +1     
=======================================
+ Hits        12354   12951   +597     
+ Misses       1630    1034   -596     

[tchaton]

LGTM !