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

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 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 37%. Comparing base (3eb05bf) to head (72d355b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #3238    +/-   ##
=======================================
- Coverage      37%     37%    -1%     
=======================================
  Files         364     351    -13     
  Lines       20096   19973   -123     
=======================================
- Hits         7520    7350   -170     
- Misses      12576   12623    +47     

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[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

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