Add Megatron-LM cross-entropy integration

[PrathyushaPolepalli]

Summary

Adds `apply_liger_kernel_to_megatron()` monkey-patch that swaps Megatron-LM's native `fused_vocab_parallel_cross_entropy` for Liger's Triton cross-entropy kernel.

  from liger_kernel.megatron import apply_liger_kernel_to_megatron                                                                                                                                                                                          
                                                                                                                                                                                                                                                            
  apply_liger_kernel_to_megatron(
      ignore_index=-100,                                                                                                                                                                                                                                    
      label_smoothing=cfg.label_smoothing_factor,                                                      
  )   

Enables online softmax + in-place gradients + no full-softmax materialization inside Megatron training pipelines.

Scope: tensor_model_parallel_size=1 only. With TP>1, each rank holds a sharded [N, V/tp] logits slice and CE requires cross-rank all-reduces that Liger's kernel does not perform.

The patch raises RuntimeError at patch time (via megatron.core.parallel_state) and again at call time (via the tp_group argument Megatron passes), so misconfiguration fails loudly. Vocab-parallel support is follow-up work.

Tested on Qwen3-30B-A3B scaled MoE, 1× H100_8, BF16:

Model config:

• 24 layers, hidden=1024, FFN hidden=6144
• 128 experts, top-8 routing, MoE FFN hidden=768
• ~7.8B total params, ~0.8B active per token
• Vocab size: 151,936
• Sequence length: 4096

Parallelism:

• Tensor Parallel (TP): 1
• Pipeline Parallel (PP): 1
• Expert Parallel (EP): 8 (16 experts per GPU)
• Data Parallel (DP): 8 (non-expert), 1 (expert)

Training config:

• Global batch size: 1024, Micro batch size: 2
• Distributed Adam optimizer
• Selective activation recompute (core_attn, mlp)
• --cross-entropy-loss-fusion enabled

Throughput results:
| Throughput | Iter time
Megatron native fused CE (baseline) | ~99 TFLOP/s/GPU | ~39,400 ms
Liger CE (this PR) | ~108 (+9%) | ~35,900 ms

Numerical correctness: lm_loss ~4.1e-3 in both, no NaN/skipped iterations.
Variance: Liger CE 107.7-109.1 TFLOP/s/GPU (consistent).

Test setup: Single H100 80GB, sequence length S=2048, batch size B=4, vocab sizes 4K → 131K. Each provider is the same cross-entropy operation, just different implementations:

• liger — apply_liger_kernel_to_megatron() patch (Liger's Triton kernel)
• torch — standard torch.nn.functional.cross_entropy
• megatron — Megatron's native fused_vocab_parallel_cross_entropy

Testing Done

• Hardware Type: H100
• run make test to ensure correctness
• run make checkstyle to ensure code style
• run make test-convergence to ensure convergence