New Metric: Upper Face Dynamics Deviation(FDD)

[VijayVignesh1]

What does this PR do?

Implements the Upper Face Dynamics Deviation (FDD) metric for 3D talking head evaluation.
Computes the deviation in motion magnitude of upper-face vertices between predicted and ground truth meshes.

Fixes #3097

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

---

📚 Documentation preview 📚: https://torchmetrics--3238.org.readthedocs.build/en/3238/

[codecov]

Codecov Report

❌ Patch coverage is 36.00000% with 48 lines in your changes missing coverage. Please review.
✅ Project coverage is 36%. Comparing base (d184220) to head (6ef4bfd).

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #3238   +/-   ##
======================================
- Coverage      37%     36%   -0%     
======================================
  Files         349     351    +2     
  Lines       19901   19973   +72     
======================================
+ Hits         7264    7288   +24     
- Misses      12637   12685   +48     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[VijayVignesh1]

@Borda The unittest error seems to coming from a timeout error from azure's end. Unrelated to the code.

[VijayVignesh1]

@Borda @rittik9 The build docs failure seems to be coming from a different broken link.

[Copilot]

Pull Request Overview

Adds the Upper Face Dynamics Deviation (FDD) metric for evaluating 3D talking head upper-face motion, including functional and class APIs, documentation, and tests.

• Introduces functional API upper_face_dynamics_deviation and class UpperFaceDynamicsDeviation
• Wires exports, adds docs and plotting test coverage, and updates changelog

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 13 comments.

Show a summary per file

File	Description
src/torchmetrics/functional/multimodal/fdd.py	Adds the functional FDD computation and docstring
src/torchmetrics/multimodal/fdd.py	Adds the metric class implementation and plotting support
src/torchmetrics/functional/multimodal/init.py	Exposes the new functional
src/torchmetrics/multimodal/init.py	Exposes the new metric class
src/torchmetrics/functional/init.py	Re-exports the functional at top-level
tests/unittests/multimodal/test_fdd.py	Adds unit tests for class, functional, errors, differentiability, and plotting
tests/unittests/utilities/test_plot.py	Adds the metric to generic plot tests
docs/source/multimodal/fdd.rst	Adds user-facing docs for the metric
docs/source/links.rst	Adds reference link for FDD
CHANGELOG.md	Notes the new metric addition

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}