Fix to Parameters to MixedPrecisionPlugin are not validated and do not match doc string

[akreuzer]

What does this PR do?

Fixes #17288

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• Did you list all the breaking changes introduced by this pull request? No breaking changes
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors) No only minor change

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[carmocca]

Glad that you found time!

We should also update:

• https://github.com/Lightning-AI/lightning/blob/master/src/lightning/fabric/plugins/precision/amp.py
• Fabric's CHANGELOG.md
• Trainer's CHANGELOG.md

[akreuzer]

Address the comments, added parameter validation to fabric (docstring is already correct there), added a line to each CHANGELOG.

[codecov]

Codecov Report

Merging #17687 (0166e1f) into master (420eb6f) will decrease coverage by 24%.
The diff coverage is 100%.

❗ Current head 0166e1f differs from pull request most recent head 017d8f0. Consider uploading reports for the commit 017d8f0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #17687     +/-   ##
=========================================
- Coverage      84%      60%    -24%     
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  Files         419      414      -5     
  Lines       31721    31583    -138     
=========================================
- Hits        26634    18810   -7824     
- Misses       5087    12773   +7686     

[carmocca]

Test failures look real:

FAILED strategies/test_fsdp.py::test_precision_plugin_config[16-true-expected2] - ValueError: `FSDPMixedPrecisionPlugin(precision='16-true')` must be '16-mix...
FAILED strategies/test_fsdp.py::test_precision_plugin_config[bf16-true-expected3] - ValueError: `FSDPMixedPrecisionPlugin(precision='bf16-true')` must be '16-m...

[awaelchli]

@carmocca 16-true and bf16-true are not yet supported in FSDPPrecision. it needs to be refactored to support this, which is why I had opened #17657