fix(qwen3-vl): per-segment mRoPE + vision under CP + THD packing

[Zhichenzzz]

Follow-up to #1272 (stacked on its branch, so this diff shows only the #1296 changes — merge after #1272).

What

Makes Qwen3-VL train end-to-end under context parallelism + THD sequence packing in bridge mode.

1. Per-segment mRoPE under CP — when the THD row is CP-sharded (zigzag), _build_packed_positions all-gathers the per-rank rows, de-interleaves to the full row (_reassemble_full_row, unit-tested in tests/fast/test_qwen3_vl_cp_mrope.py), rebuilds per-segment MRoPE, and re-slices into this rank's zigzag layout.
2. Don't double-shard — when the input is already CP-local, the bridge's internal preprocess_packed_seqs is wrapped to an identity that returns miles' full-cu packed_seq_params (CP attention still sees the full cu; the data isn't re-split).
3. CP-local vision embeds — select_local_vision_embeds maps each rank's local vision tokens to the matching slice of the full vision-tower output (and deepstack). Cooperates with a small hook in megatron-bridge (separate PR to radixark/Megatron-Bridge).
4. Wires calculate_per_token_loss into the bridge provider (Qwen3-VL asserts on it under CP) and a defensive AllGatherVisionEmbeddings.apply kwarg shim.

Validation (e2e)

Qwen3-VL-2B geo3k, CP=2 TP=4, 8×H200, THD packed, bridge mode: stable over 3 steps, train_rollout_logprob_abs_diff 0.0141 → 0.0146 (healthy, == non-CP 0.011–0.016), rollout/raw_reward ~0.4, no crashes.

Depends on

• feat: Fix Qwen3-VL THD packed mRoPE positions #1272 (base of this PR)
• megatron-bridge vision hook (next PR to radixark/Megatron-Bridge) — until it lands, the miles-side monkeypatch provides the hook.

Fixes #1296

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Update: cleanup pass + re-validation

• helpers' first parameter renamed self → model (they are free functions, not methods)
• the preprocess_packed_seqs identity wrapper forwards *args/**kwargs instead of hard-coding the upstream signature
• the patch warns once at install when the running megatron-bridge lacks the _miles_select_local_vision_embeds hook (instead of silently mis-placing vision embeddings under CP); points at the matching Megatron-Bridge patch

Re-validated after cleanup: unit tests 6/6; Qwen3-VL-2B CP2 TP4 THD geo3k RL — train_rollout_logprob_abs_diff 0.0127–0.0131 (same healthy band as the original validation), coherent rollouts, no hook warning with the bridge patch installed.

Note on stacking: this branch contains #1272; the cleanup lives here because this PR owns the final shape of qwen3_vl_packed_mrope.py.

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Update: vision-embed plumbing removed (−53 lines)

Megatron-Bridge PR #9 now selects CP-local vision embeddings natively inside Qwen3VLModel (no override hook), so this PR no longer carries select_local_vision_embeds, the local→full position mapping, or any hook installation. What remains on the miles side: per-segment mRoPE position reconstruction and the preprocess_packed_seqs identity wrapper (both still needed — they are about positions, not embeddings), plus a warning when the running bridge lacks the native support.

Re-validated end-to-end after the removal (Qwen3-VL-2B, CP2 TP2, THD, geo3k RL): train_rollout_logprob_abs_diff 0.0130, coherent rollouts, no warnings, no crashes.

[gemini-code-assist]

Code Review

This pull request introduces support for Qwen3-VL context parallelism (CP) and THD packed mRoPE reconstruction. It adds patches to handle CP-local vision embeddings, bypasses redundant re-sharding in preprocess_packed_seqs, and implements a shim for AllGatherVisionEmbeddings to accept cp_group as a keyword argument. Additionally, a comprehensive CPU unit test suite is added to verify the correctness of the zigzag reconstruction. The reviewer feedback suggests improving the robustness of the patches by having the _AllGatherVisionEmbeddingsKwargShim inherit from the original class to preserve its type hierarchy, and using *args and **kwargs in the preprocess_packed_seqs wrapper to guard against future signature changes.

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[gemini-code-assist]

To ensure that _AllGatherVisionEmbeddingsKwargShim behaves identically to the original AllGatherVisionEmbeddings class (e.g., preserving class attributes, static methods, or satisfying issubclass / isinstance checks in downstream code), it is safer to have the shim inherit from orig instead of being a completely separate, plain class.

Suggested change

	class _AllGatherVisionEmbeddingsKwargShim:
	_miles_kwarg_shim = True
	@staticmethod
	def apply(input, seqlens_on_cp_ranks, cp_group=None):
	return orig.apply(input, seqlens_on_cp_ranks, cp_group)
	class _AllGatherVisionEmbeddingsKwargShim(orig):
	_miles_kwarg_shim = True
	@staticmethod
	def apply(input, seqlens_on_cp_ranks, cp_group=None):
	return orig.apply(input, seqlens_on_cp_ranks, cp_group)

[gemini-code-assist]

To make the monkeypatched wrapped function more robust against future signature changes in preprocess_packed_seqs (e.g., if the bridge library adds or reorders arguments), it is highly recommended to use *args and **kwargs and extract input_ids dynamically. This prevents potential TypeError exceptions due to signature mismatches.

Suggested change

	def wrapped(input_ids, attention_mask, pre_process=True, pg_collection=None):
	ctx = getattr(_tls, "cp_local", None)
	if ctx is not None:
	# already-local CP path: do not re-shard; return the data unchanged together with
	# miles' full-cu packed_seq_params (callers ignore the psp; the model's CP attention
	# uses the packed_seq_params passed into forward, which already has the full cu).
	return input_ids, ctx["psp"]
	return orig(input_ids, attention_mask, pre_process=pre_process, pg_collection=pg_collection)
	def wrapped(*args, **kwargs):
	ctx = getattr(_tls, "cp_local", None)
	if ctx is not None:
	# already-local CP path: do not re-shard; return the data unchanged together with
	# miles' full-cu packed_seq_params (callers ignore the psp; the model's CP attention
	# uses the packed_seq_params passed into forward, which already has the full cu).
	input_ids = kwargs.get("input_ids") if "input_ids" in kwargs else args[0]
	return input_ids, ctx["psp"]
	return orig(*args, **kwargs)