Megatron: Nemotron RL support - #51
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June 10, 2026 06:52
Squashed transplant of mtp-hybrid + cleanup pass: - Design mirrors nvidia/main (parse_hybrid_pattern, mtp_model_layer rename, is_mtp_layer plumbing, standalone process_mtp_loss, MTP FLOPs accounting). - Fixes the routing-replay MTP bypass regression (is_mtp_layer into the R3 gate; the old Router.is_mtp flag had lost its only writer); removes dead set_is_mtp; upstream-form isinstance attn-mask validation; deduped arguments asserts; zero reformat-only churn. - Carries the make_viewless_tensor dead-store fix and the vocab_size test typo fix. - Rebased onto current miles-main (router layer_number / tid2eid / R3 input_ids co-exist with is_mtp_layer).
The bridge/mamba path lacks the GPT provider's enable/disable dance around MTP block construction, so MTP MoE routers registered replay slots that the trainer-side fill (which only records main-decoder MoE layers) never populates -- a main layer then pops an empty slot (IndexError: len(top_indices_list)=0). Gate registration on is_mtp_layer, mirroring the existing is_mtp bypass in topk routing.
The MambaStack layer builder passed is_mtp_layer to attention layers but not to MoE layers, so MTP-block MoE routers saw is_mtp_layer=False -- defeating both the rollout-replay slot-registration skip and the is_mtp topk bypass (the slot/fill desync then popped an empty replay: IndexError len(top_indices_list)=0).
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radixark/miles#1284