Fix type check for non-standard schedulers in horovod

[awaelchli]

What does this PR do?

Fixes #14236

Fixes a typing assertion error being caused when using a special scheduler like ReduceOnPlateau with horovod.
Discovered in a question on stackoverflow.

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @awaelchli

[codecov]

Codecov Report

Merging #14215 (ac7c846) into master (52051c5) will decrease coverage by 3%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #14215      +/-   ##
==========================================
- Coverage      79%      76%      -3%     
==========================================
  Files         109      330     +221     
  Lines        7149    26760   +19611     
==========================================
+ Hits         5662    20342   +14680     
- Misses       1487     6418    +4931     

[akihironitta]

Thank you! LGTM!