Add explicit comment/warning to training_epoch_end()

[ashleve]

What does this PR do?

Adds explicit comment/warning to training_epoch_end() hook.

Fixes #458

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[codecov-commenter]

Codecov Report

Base: 84.46% // Head: 84.46% // No change to project coverage 👍

Coverage data is based on head (8b71c3f) compared to base (75b44ff).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #486   +/-   ##
=======================================
  Coverage   84.46%   84.46%           
=======================================
  Files           9        9           
  Lines         354      354           
=======================================
  Hits          299      299           
  Misses         55       55           

Impacted Files	Coverage Δ
src/models/mnist_module.py	98.41% <ø> (ø)

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