[DPO] Add hinge, bco_pair, robust, exo_pair, discopop loss types

[kashif]

Brings Liger's fused linear DPO loss closer to TRL's DPOTrainer by adding five additional loss variants on top of the existing sigmoid / apo_zero / apo_down / sppo_hard / nca_pair set:

• hinge: relu(1 - beta * (chosen - rejected))
• bco_pair: -logsigmoid(beta * chosen) - logsigmoid(-beta * rejected)
• robust (cDPO): label-smoothed sigmoid loss with flipped-pair correction
• exo_pair: KL(p_fθ || [1-eps, eps]) for K=2 (EXO-pref, paper 2402.00856)
• discopop: blended logistic / exponential loss (DiscoPOP, paper 2406.08414)

Also threads two new kwargs through LigerFusedLinearDPOLoss / LigerFusedLinearDPOFunction:

• label_smoothing (used by robust and exo_pair; validated at construction)
• discopop_tau (temperature for the DiscoPOP modulation term)

Tests: adds HF reference implementations for each new loss type and a parametrized test_correctness_extra_loss_types covering all five variants across bf16/fp32, with/without bias, with/without ref bias, and compute_nll_loss on/off (320 cases). Adds test_label_smoothing_validation for the label_smoothing range checks. The existing test_invalid_loss_type is updated to cover the new supported set.

All 800 DPO test cases pass on H100.

Summary

Testing Done

• Hardware Type:
• run make test to ensure correctness
• run make checkstyle to ensure code style
• run make test-convergence to ensure convergence