Fix: TorchMetrics documentation source link

[bhimrazy]

What does this PR do?

This PR updates the source link for TorchMetrics docs from https://lightning.ai/docs/torchmetrics to https://lightning.ai/docs/torchmetrics/stable at docs/source-pytorch/conf.py, so that it redirects correctly .

Fixes:

(extensions/logging: line  125) broken    https://lightning.ai/docs/torchmetrics/v1.8.1//stable/pages/lightning.html - 404 Client Error: Not Found for url: https://lightning.ai/docs/torchmetrics/v1.8.1/stable/pages/lightning.html

found in #21103

Before submitting

• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure 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? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

Reviewer checklist

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified