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.

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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

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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     

[le-Greg]

How to get the old behaviour now ? I would like to keep my models stored under /models, now they are logged in the root /, which is a bit messy. I tried artifact_location but it doesn't change anything, and save_dir doesn't work since i am using a tracking uri