training loop refactor - move val loop

[awaelchli]

What does this PR do?

We currently have the validation loop owned by the fit loop, i.e.,

trainer.fit_loop.val_loop: EvaluationLoop

It was introduced there in #8065.
However, the validation loop is never used in the fit loop. It is called only in the epoch loop, i.e.
trainer.fit_loop.epoch_loop calls the validation loop at the right interval. This PR moves the val loop
from

trainer.fit_loop.val_loop to
trainer.fit_loop.epoch_loop.val_loop

This does not require a changelog entry, since we are developing these loops on master only at the moment.

Discussed with @tchaton and @carmocca so far.

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[codecov]

Codecov Report

Merging #8120 (2d04145) into master (b12a0d0) will decrease coverage by 5%.
The diff coverage is 88%.

@@           Coverage Diff           @@
##           master   #8120    +/-   ##
=======================================
- Coverage      93%     88%    -5%     
=======================================
  Files         212     212            
  Lines       13549   13541     -8     
=======================================
- Hits        12590   11897   -693     
- Misses        959    1644   +685     

[tchaton]

LGTM !