[None][feat] Support the masked DSA indexer k-cache pool in the Python cache transceiver - #17283

Description

The Python (v2) KV-cache transceiver previously raised NotImplementedError for the per-layer masked DSA indexer k-cache pool (cross-layer indexer sharing, e.g. GLM 5.2), so those checkpoints were forced onto the C++ transceiver — #16558 added a GlmMoeDsaForCausalLM -> CPP preference as a stop-gap while the Python path lacked this support.

This PR teaches the Python transceiver to handle the masked layout, mirroring the C++ support added in #16558:

• build_page_table (tensorrt_llm/_torch/disaggregation/resource/kv_extractor.py): the indexer REPLICATED pool view now covers only the indexer-owning layers — one buffer_entries row per owning layer, each mapped to its packed pool row via impl.get_indexer_k_cache_pool_layer_idx(lid) — and skips the indexer pool entirely for a layer group with no owning layers (which would otherwise hit the null-pool getter). The dense/unmasked layout is byte-for-byte unchanged.
• No transfer-machinery changes are needed: it already matches peers per-pool by pool_role + global_layer_id overlap, so a masked subset transfers correctly, including under PP resharding (the generic analogue of the C++ indexerLayerNumPerPP / targetIRanksForIndexerKCache interval logic).
• With Python support in place, GlmMoeDsaForCausalLM prefers the Python transceiver again like the other DeepSeek-family checkpoints, reverting the CPP override that [None][perf] Allocate DSA indexer k-cache only for layers that own an indexer #16558 added only because Python lacked masked-pool support.

Test Coverage

• tests/unittest/disaggregated/test_extractor.py::test_v1_dsa_masked_indexer_page_table_covers_owning_layers (new) — builds a V1 KVCacheManager with a per-layer indexer mask ([True, False, True, False]) and asserts the indexer view is REPLICATED, covers exactly the two owning layers, and maps them to the correct packed pool rows/offsets.
• test_extractor.py::test_v1_dsa_indexer_page_table_is_replicated_with_per_layer_entries and ::test_v1_dsa_indexer_replicated_transfer_across_pp (existing) — continue to guard the dense layout and the end-to-end replicated transfer path.
• tests/unittest/llmapi/test_llm_args.py::TestDeepseekTransceiverPreference::test_preference_per_architecture — updated to expect PYTHON for glm_moe_dsa again.

PR Checklist

• PR description clearly explains what and why.
• PR follows TRT-LLM coding guidelines.
• Test cases are provided for new code paths.
• No API changes (the affected get_preferred_transceiver_runtime is an internal preference hook).

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Dev Engineer Review

• build_page_table now supports masked DSA indexer K-cache pools.
• It creates REPLICATED views only for indexer-owning layers.
• It maps each layer to its packed pool row.
• It omits fully masked layer groups.
• Dense and unmasked layouts remain unchanged.
• Existing transfer logic supports masked subsets and pipeline-parallel resharding.
• GlmMoeDsaForCausalLM now prefers the Python transceiver.
• No public API, configuration, or test-list changes were identified.
• The implementation should be checked against CODING_GUIDELINES.md, with focus on offset mapping, empty-group handling, and regression coverage for dense layouts.

QA Engineer Review

• Modified test coverage includes:
  • _make_v1_dsa_manager, updated to accept an indexer K-cache mask.
  • A CUDA test for masked replicated indexer page-table construction.
  • DeepSeek transceiver preference tests, updated for Python runtime selection.
• The tests cover masked page-table entries, packed offsets, dense transfer behavior, cross-pipeline transfer, and architecture preference.
• No corresponding test-db/ or qa/ coverage was identified in the provided changes.
• Verdict: needs follow-up.