Fix metrics logged as percentage in wandb/tensorboard/jsonl

[youssefbench]

What does this PR do? Please describe:

Metrics using format_as_percentage (padding_ratio, peak_active_mem_ratio, peak_reserved_mem_ratio) were logged as raw 0-1 floats to numeric backends (wandb, TensorBoard, JSONL) while the console logger correctly displayed them as percentages. This happened because the formatter returns a string ("42%") which only the console logger uses — numeric backends skip it entirely and pass raw values through.

Fix

The fix adds an optional value_transform field to MetricDescriptor that applies a numeric transformation before logging. A new scale_as_percentage function handles int, float, and Tensor types by multiplying by 100. The three numeric recorders (wandb, TensorBoard, JSONL) now apply this transform when present, while the console logger continues using the string formatter.

Also changes format_as_percentage to display 2 decimal places ("42.26%" instead of "42%").

Changes:

• Add MetricValueTransform protocol and value_transform field to MetricDescriptor
• Add scale_as_percentage function to formatters
• Apply value_transform in WandbRecorder, TensorBoardRecorder, JsonlMetricRecorder
• Register value_transform=scale_as_percentage for the 3 percentage metrics
• Update format_as_percentage to use 2 decimal places
• Add 25 unit tests covering formatters and all 4 recorders

Does your PR introduce any breaking changes? If yes, please list them:
None

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)