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Hybrid quantization follow-ups

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Dal 30/6/2026.

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Descrizione

bug

Tracking follow-ups from PR #2817. Detailed implementation notes are already in code comments / xfails.

Recipe / validation

  • Validate per-GEMM scaling-mode compatibility for hybrid qfactories.
    • See HybridQuantizer._get_compatible_recipe() in transformer_engine/pytorch/tensor/hybrid_tensor.py.
  • Support delayed-scaling requests inside HybridQuantizer.
    • See HybridQuantizer.__init__() in `transformer_engine/pytorch/tensor/hybrid_tensor.py
  • Support producing both directional representations at the fused normalization boundary (transformer_engine/pytorch/ops/basic/layer_norm.py)

Performance / kernels

  • Fused kernels: one read -> two writes.
  • Introduce a policy for when to materialize a tensor for columnwise_source="rowwise_dequantized".
  • Support module-/role-specific CustomRecipe quantization alignment. The current recipe-global quantization_alignment must use the maximum requirement across all factory outputs, which can overpad lower-alignment MXFP8/FP8 layers in mixed-format models. Derive alignment from canonical cached module quantizers and propagate it to the corresponding Fp8Padding without speculative qfactory calls.
  • Support HybridQuantizer with standard Userbuffers.
  • Add native columnwise-only per-tensor FP8 quantization for Hopper, covering both CurrentScaling and DelayedScaling. The legacy per-tensor cast-transpose kernel currently requires both rowwise and columnwise output buffers.

TP/SP

  • Add native HybridQuantizer dispatch to gather_along_first_dim.
  • Preserve existing SP amax-reduction semantics and cover them in distributed tests.

FSDP2

  • Optimize hybrid FSDP2 communication buffers.
    • See HybridQuantizedTensor.fsdp_pre_all_gather() in transformer_engine/pytorch/tensor/hybrid_tensor.py.
  • Fix HybridFloat8BlockScaling FSDP2 xfail.
    • See _HYBRID_FLOAT8_BLOCK_FSDP2_XFAIL_REASON in tests/pytorch/distributed/fsdp2_tests/conftest.py.
  • Add NVFP4 hybrid sub-storage FSDP2 hooks.
    • See TestHybridFsdpPreAllGatherProtocol.test_nvfp4_sub_storage_raises_on_pre_all_gather() in tests/pytorch/test_hybrid_quantization.py.
    • Non-hybrid FSDP alignment: reuse fsdp extract buffers and fsdp_assign_gather for the current base tensor class (non-hybrid)
  • Support Hybrid sub-storages that fall back to a high-precision tensor for an unquantizable local shard (TransformerEngine/transformer_engine/pytorch/tensor/hybrid_tensor.py).

GEMM / quantization

  • Support HybridQuantizer / IdentityQuantizer as GEMM output quantizers.
    • See _reject_unsupported_output_quantizer() in transformer_engine/pytorch/cpp_extensions/gemm.py.

GroupedLinear / grouped storage

  • Support IdentityQuantizer and HybridQuantizer with GroupedLinear(single_grouped_weight=True).

Distributed optimizer / Megatron

  • Support per-block hybrid sub-quantizers in quantize_master_weights.
  • Hybrid distributed optimizer: partial-master support with columnwise_source="original". The current one-payload distributed-optimizer path cannot preserve both independently quantized Hybrid directions when each rank owns only a partial master weight. A safety guard rejects this configuration unless full-master data or an FSDP Hybrid shard containing both directions is provided. Unblock by adding a two-payload sharding/all-gather contract for Hybrid rowwise and columnwise storage, including their scale/amax metadata, then relax _validate_hybrid_partial_master_policy.
  • Hybrid distributed optimizer with columnwise_source="rowwise_dequantized": reconstruct the column only after the rowwise update/all-gather.
  • Complete Megatron-LM quantized_model_init + --fp{4,8}-param-gather + dist opt.
    • See PR #2817 integration notes.
  • Complete Megatron-FSDP + --fp{4,8}-param-gather.
    • See PR #2817 integration notes.
  • Complete Torch FSDP2 + --fp{4,8}-param-gather.
    • See PR #2817 integration notes and tests/pytorch/distributed/fsdp2_tests/.

Activation recompute

  • Investigate vanilla torch.utils.checkpoint(use_reentrant=False) with TE weight-workspace cache.
    • See xfails in TestHybridActivationRecompute in tests/pytorch/test_hybrid_quantization.py.
    • te.checkpoint path is already covered and works.

Validation

  • Convergence validation of base non-hybrid recipes.
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