Fix tbptt_reduce_fx when non-floating tensors are logged

[carmocca]

What does this PR do?

Fixes #3778

Questions:

• Not sure if that is the best file for the test
• See TODO at the end of the test

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together? Otherwise, we ask you to create a separate PR for every change.
• [n/a] Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• [n/a] If you made a notable change (that affects users), did you update the CHANGELOG?

[Borda]

@carmocca nice of test-driven, starting with failing test... :]

[williamFalcon]

i'm finishing this

[codecov]

Codecov Report

Merging #3796 into master will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #3796   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         117     117           
  Lines        9107    9107           
======================================
+ Hits         7966    7967    +1     
+ Misses       1141    1140    -1