feat: add TripletMarginLoss

[Bellman281]

Checklist

• Confirmed that cargo run-checks command has been executed.
• Made sure the book is up to date with changes in this PR.

Related Issues/PRs

Part of the PyTorch loss-parity effort, following SoftMarginLoss (#5149),
MarginRankingLoss (#5151), and HingeEmbeddingLoss.

Changes

Adds TripletMarginLoss / TripletMarginLossConfig, matching torch.nn.TripletMarginLoss:
for each triplet, max(0, ||a − p||_p − ||a − n||_p + margin) (defaults margin = 1.0,
p = 2.0). Distances use burn::tensor::linalg::lp_norm, and the loss composes existing
ops so autodiff is free. Follows the MarginRankingLoss config/module pattern; added to the
book's loss table.

Testing

Known-answer test over forward_no_reduction, Mean, and Sum against PyTorch reference
values (3 triplets exercising a normal, a partial, and a clamped-to-zero case), plus a display
test. cargo run-checks passes.

[codecov]

Codecov Report

❌ Patch coverage is 98.71795% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 66.11%. Comparing base (12cba54) to head (400f6b5).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/burn-nn/src/loss/triplet_margin.rs	98.71%	1 Missing ⚠️

❌ Your project check has failed because the head coverage (66.11%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5175      +/-   ##
==========================================
+ Coverage   66.03%   66.11%   +0.07%     
==========================================
  Files        1206     1217      +11     
  Lines      183117   183581     +464     
==========================================
+ Hits       120927   121371     +444     
- Misses      62190    62210      +20     

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