Decoupled checkpoint artifact path from model artifact path

[pete-mcelroy]

What does this PR do?

Addresses the issues described in issue 19394 by making the checkpoint artifact path in the MLFlowLogger class fully customizable by removing the enforced "model/checkpoints" prefix.

As shown in the image below, we can now specify an artifact path for our checkpoints outside the model/ subfolder. This means that when we want to load a model, we do not have to load all checkpoints at the same time.

Fixes #19394

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81%. Comparing base (5dea36c) to head (9e94053).
Report is 1 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (5dea36c) and HEAD (9e94053). Click for more details.

HEAD has 554 uploads less than BASE

Flag	BASE (5dea36c)	HEAD (9e94053)
cpu	148	21
python3.10	42	6
lightning_fabric	25	0
pytest	87	2
lightning	107	16
python3.9	43	6
python3.11	42	6
gpu	3	2
python3.12	21	3
pytorch2.1	37	12
pytest-full	64	21
pytorch2.3	9	3
pytorch_lightning	19	7
pytorch2.2	9	3
pytorch2.4	9	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20325     +/-   ##
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- Coverage      88%      81%     -7%     
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  Files         267      264      -3     
  Lines       23065    23010     -55     
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- Hits        20277    18606   -1671     
- Misses       2788     4404   +1616