Improve ConFIG implementation

[PierreQuinton]

• Improve implementation of the matrix-vector product to simply use a single @.
• Change the vector normalization to use torch.nn.functional.normalize with the default epsilon of 1e-12. This should change the output of the aggregator for very uncertain vectors to be zero instead.
• Add changelog entry

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/torchjd/aggregation/config.py	100.00% <100.00%> (ø)

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[ValerianRey]

GPU tests all pass.