Mask padded video frames from attention

CHANGELOG.md

@@ -65,7 +65,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - **Video data path (pluggable, registry-driven).** `kempnerforge/data/video_io.py`: timestamp-based frame sampling (target fps, first & last frame kept — Molmo2 §3.1/§A) registered as the `"uniform"` sampling policy and selectable via `[video].sampling_policy`; PyAV decode (lazy-imported). `kempnerforge/data/video_dataset.py`: a `VideoDataset` base + the WebVid-style `WebVidVideoDataset` (CSV manifest + `id[:2]/id[:4]/id[:6]/id.mp4` mapping) registered as `"webvid"` via `@registry.register_video_dataset`, plus a `build_video_dataset` dispatch — so other dataset styles are additive registrations selected by `[video].dataset_type`. The WebVid corpus directory is parameterized by `[video].dataset_name` (no longer hardcoded to `webvid-10M`). `VideoCollator` → `(B, F, 3, H, W)` + a frame-validity mask; an undecodable clip is masked out (no loss). `kempnerforge/config/registry.py`: `register_video_dataset` / `register_sampling_policy` registries. `kempnerforge/config/video.py`: the `[video]` `VideoConfig` section (`data_root`, `dataset_type`, `dataset_name`, `sampling_policy`, `split`, `fps`, `max_frames`, `min_frames`, `frame_size`, `max_samples`), wired into `JobConfig` (+ `is_video`). `av` is an optional `video` dependency group (`uv sync --group video`); CI installs it for the lint + unit-test jobs.
   - **Frame-aware model + training wiring.** `kempnerforge/model/vlm.py`: `_project_image_features` → `_project_visual_features` folds the frame axis through the encoder + pooler to `(B, F·P′, dim)` (a single image is the `F == 1` case). `VLMWrapper` gains `frames_per_clip`, threaded through `build_parallel_model` / `_build_vlm` / `build_vlm_wrapper` so the static visual-token count equals `F·P′` (drives the residual budget and MoT's positional split; static == runtime). `scripts/train.py` builds the video dataset/collator when `[video]` is set. Adds `configs/train/vlm_video_webvid.toml` (SigLIP2 + avgpool + WebVid).
   - Tests: `tests/unit/test_video_io.py`, `test_video_dataset.py`, `test_video_config.py`; video-forward cases (all four archs) + image-path regression in `test_vlm.py`; pooling-adapter cases in `test_adapter.py`. Docs: `docs/how-to/train-on-video.md`.
-  - Deferred (follow-ups; the registries make these additive): more video dataset styles (HuggingFace video sets, flat folders, alternate manifests) and frame-sampling policies; per-frame timestamp tokens + grounding (`<points>`/`<tracks>` outputs with point-F1 / track-J&F eval), frame-mask-aware attention, bidirectional visual attention, VLM sequence packing, long-context (blocked on context-parallel being wired), and warm-start from a converted image-VLM checkpoint.
+  - Deferred (follow-ups; the registries make these additive): more video dataset styles (HuggingFace video sets, flat folders, alternate manifests) and frame-sampling policies; per-frame timestamp tokens + grounding (`<points>`/`<tracks>` outputs with point-F1 / track-J&F eval), bidirectional visual attention, VLM sequence packing, long-context (blocked on context-parallel being wired), and warm-start from a converted image-VLM checkpoint.
+- **Padded frames masked from attention (all four archs).** Short/undecodable video clips pad to `max_frames` with blank frames; the `frame_mask` is now consumed so real tokens never attend to padded-frame visual tokens. One per-token validity mask, `ModalityContext.key_padding_mask` `(B, S)`, threads through the model: the shared `Attention` ANDs it with the causal (and doc) mask — covering Joint-Decoder and MoMa — `MoTAttention` builds an explicit causal-AND-valid mask, and Cross-Attention masks the padded image K/V via its existing `image_mask`. A NaN guard unmasks fully-masked query rows (an all-padded clip) so softmax stays finite. It is a **pure mask — no new state-dict keys**, so checkpoints stay compatible both ways; the image (`F=1`) and text paths are unchanged (no mask is built, so they keep the FlashAttention-2 path). Note: for the image-prefix arches (Joint-Decoder/MoT/MoMa) video self-attention always takes the explicit-mask SDPA path (FA2 disabled, a `(B,1,S,S)` mask materialized) even for fully-decoded clips — the result is identical to causal-only but not free; a deliberate `torch.compile`/DP-friendly trade-off (one graph, no host sync), with FA2-recovery / FlexAttention left as a follow-up. (Cross-Attention keeps FA2 on its text self-attention and only masks padded image K/V in the cross-attention blocks.) Foundation for variable-length / mixed image+video batches.
+  - `kempnerforge/model/modality.py`: `ModalityContext.key_padding_mask` field (+ invariant). `kempnerforge/model/vlm.py`: `_visual_token_mask` expands `frame_mask (B,F)` → `(B, F·P′)`; the four strategies place it (image-prefix arches → `key_padding_mask`, Cross-Attention → `image_mask`). `attention.py` / `mot.py` / `cross_attention.py` consume the mask (+ NaN guard); `moma.py`'s `MoMaFFN` also excludes padded positions from expert-choice routing (so padded tokens never consume expert capacity). `scripts/train.py` threads `batch["frame_mask"]` into the forward.
+  - Deferred: MoT configured with an *MoE* FFN still routes padded tokens through the shared `MoEMLP` — a follow-up ("generic token-validity in MoE") would mask that and padded text alike. MoT-dense (the default) and MoMa are fully masked.
+  - Tests: `tests/unit/test_vlm.py` (per-arch masking invariance, image no-op, undecodable-clip NaN guard, mask expansion); `test_moma.py` (FFN routing exclusion); `test_modality_context.py` (the new invariant).
 - `install-and-verify` plugin skill: runs `uv sync`, asserts Python ≥ 3.12, then runs the four CI gate checks (`ruff check`, `ruff format --check`, `pyright`, `pytest tests/unit/`). Canonical first command after cloning.
 - `.python-version` pinned to `>=3.12` so uv resolves the interpreter explicitly. Teammates on 3.13 use 3.13 (no download); 3.11-only users get 3.12 auto-fetched.
 - **Dynamic-checkpointing window** (`[checkpoint.dyn_ckpt_window]`). Opt-in dense save phase: inside `[start, stop]` a registered strategy decides which steps to save; outside the window the regular `interval` cadence applies. The default strategy, `"power2"`, saves at `start` and at every `start + 2^k` while `<= stop` — tight near the start of the window, doubling thereafter. Useful for analyzing early-training dynamics, where the loss moves fastest. The default `CheckpointConfig` is unchanged (no `dyn_ckpt_window`, interval-only saves).

docs/how-to/train-on-video.md

@@ -102,9 +102,19 @@ time, so it is set in the TOML, not via a `--vlm.arch=` CLI override.)
 - **Causal attention; no per-frame timestamps yet** — temporal order is frame
   order. Per-frame timestamp tokens + grounding (`<points>`/`<tracks>` outputs
   with point-F1 / track-J&F eval) are a follow-up.
-- **Padded frames are not yet masked from attention** — short clips pad to
-  `max_frames` with blank frames; a `frame_mask` is produced but not yet
-  consumed by the attention mask.
+- **Padded frames are masked from attention** — short/undecodable clips pad to
+  `max_frames` with blank frames, and the `frame_mask` is consumed so real
+  tokens never attend to padded-frame visual tokens (MoMa also drops them from
+  expert-choice routing); a NaN guard keeps an all-padded clip finite. It is a
+  pure mask (no new checkpoint keys); image/text keep the FlashAttention-2 path.
+  For the image-prefix arches (Joint-Decoder/MoT/MoMa), video self-attention
+  always takes the explicit-mask SDPA path (FA2 disabled, a `(B,1,S,S)` mask
+  built) even for fully-decoded clips — a deliberate compile/DP-friendly
+  trade-off; recovering FA2 / FlexAttention is a follow-up. (Cross-Attention
+  keeps FA2 on its text self-attention; it masks padded image K/V in the
+  cross-attention blocks instead.) *Remaining:* MoT
+  configured with an MoE FFN still routes padded tokens through the shared MoE
+  (a "generic token-validity in MoE" follow-up).
 - **Fixed `F` per batch** keeps tensor shapes static (for `torch.compile` and
   DP-rank consistency); variable-length clips arrive with VLM sequence packing.
 - **Long-context** (many frames) is blocked on context-parallel being wired.

kempnerforge/model/attention.py

@@ -121,6 +121,7 @@ def forward(
         *,
         kv_cache: KVCache | None = None,
         doc_ids: torch.Tensor | None = None,
+        key_padding_mask: torch.Tensor | None = None,
     ) -> torch.Tensor:
         """Forward pass.
 
@@ -132,6 +133,11 @@ def forward(
             doc_ids: Optional per-token document IDs for packed sequences,
                 shape (batch, seq_len). When provided, constructs a block-diagonal
                 causal mask so tokens only attend within their document.
+            key_padding_mask: Optional per-key validity mask, shape
+                (batch, seq_len); ``True`` = attend, ``False`` = drop (e.g. the
+                visual tokens of padded video frames). Combined with the causal
+                (and doc) mask; fully-masked query rows are unmasked to keep
+                softmax finite.
 
         Returns:
             Output tensor of shape (batch, seq_len, dim).
@@ -177,14 +183,46 @@ def forward(
         #    restrict attention to only the first key position).
         if self.capture_attention_weights:
             # Manual attention for weight extraction (analysis only, not for training)
-            out, attn_weights = self._attention_with_weights(q, k, v, seq_len, doc_ids, kv_cache)
+            out, attn_weights = self._attention_with_weights(
+                q, k, v, seq_len, doc_ids, kv_cache, key_padding_mask
+            )
             self.last_attention_weights = attn_weights.detach().cpu()
-        elif doc_ids is not None:
+        elif doc_ids is not None or key_padding_mask is not None:
+            # An explicit attn_mask is not a FlashAttention-2 shape, so SDPA falls
+            # back to the mem-efficient/math kernel here. The image-prefix video
+            # arches (Joint-Decoder/MoMa here, MoT in mot.py) always pass a
+            # key_padding_mask (all-True when unpadded), so their self-attention
+            # always takes this branch -- losing FA2 and materializing a (B, 1, S, S)
+            # mask even for fully-decoded clips. (Cross-Attention sets no
+            # key_padding_mask on this text self-attention -- it masks padded image
+            # K/V in the cross-attention blocks instead -- so it keeps FA2 here.)
+            # Deliberate: always-masking is torch.compile / DP-friendly (one graph,
+            # no host sync). Recovering FA2 for unpadded batches (or moving to
+            # FlexAttention) is a follow-up.
+            #
+            # Asserts no kv_cache: neither doc_ids (packed training) nor
+            # key_padding_mask (VLM video) co-occurs with decode today, and this
+            # branch's full-sequence causal mask would mis-handle a cached
+            # (seq_len=1) decode rather than attend to all cached positions.
+            assert kv_cache is None, (
+                "doc_ids / key_padding_mask are not supported with kv_cache decode "
+                "(would build an incorrect causal mask)."
+            )
             seq_len_kv = k.shape[2]
-            # Block-diagonal mask: same-document AND causal
-            doc_mask = doc_ids.unsqueeze(2) == doc_ids.unsqueeze(1)  # (B, S, S)
+            # Explicit bool mask: causal, AND same-document (doc_ids), AND valid
+            # keys (key_padding_mask, e.g. dropping padded video frames' tokens).
             causal = torch.ones(seq_len, seq_len_kv, dtype=torch.bool, device=q.device).tril()
-            attn_mask = (doc_mask & causal).unsqueeze(1)  # (B, 1, S, S)
+            attn_mask = causal.unsqueeze(0).unsqueeze(0)  # (1, 1, S, S_kv)
+            if doc_ids is not None:
+                doc_mask = (doc_ids.unsqueeze(2) == doc_ids.unsqueeze(1)).unsqueeze(1)  # (B,1,S,S)
+                attn_mask = attn_mask & doc_mask
+            if key_padding_mask is not None:
+                attn_mask = attn_mask & key_padding_mask.view(batch, 1, 1, seq_len_kv)
+            # NaN guard: a query row with no reachable valid key (e.g. the leading
+            # positions of an all-padded / undecodable clip) would softmax over all
+            # -inf -> NaN. Unmask such rows; their outputs are discarded (trimmed by
+            # output_slice, or the clip's labels are all -100).
+            attn_mask = attn_mask | ~attn_mask.any(dim=-1, keepdim=True)
             with self._sdpa_context():
                 out = F.scaled_dot_product_attention(q, k, v, attn_mask=attn_mask)
         else:
@@ -205,6 +243,7 @@ def _attention_with_weights(
         seq_len: int,
         doc_ids: torch.Tensor | None,
         kv_cache: KVCache | None,
+        key_padding_mask: torch.Tensor | None = None,
     ) -> tuple[torch.Tensor, torch.Tensor]:
         """Compute attention output and weights manually (for analysis).
 
@@ -221,11 +260,15 @@ def _attention_with_weights(
         attn = torch.matmul(q, k.transpose(-2, -1)) * scale
 
         seq_len_kv = k.shape[2]
-        if doc_ids is not None:
-            doc_mask = doc_ids.unsqueeze(2) == doc_ids.unsqueeze(1)
+        if doc_ids is not None or key_padding_mask is not None:
             causal = torch.ones(seq_len, seq_len_kv, dtype=torch.bool, device=q.device).tril()
-            mask = ~(doc_mask & causal).unsqueeze(1)
-            attn = attn.masked_fill(mask, float("-inf"))
+            valid = causal.unsqueeze(0).unsqueeze(0)  # (1, 1, S, S_kv)
+            if doc_ids is not None:
+                valid = valid & (doc_ids.unsqueeze(2) == doc_ids.unsqueeze(1)).unsqueeze(1)
+            if key_padding_mask is not None:
+                valid = valid & key_padding_mask.view(q.shape[0], 1, 1, seq_len_kv)
+            valid = valid | ~valid.any(dim=-1, keepdim=True)  # NaN guard (see forward)
+            attn = attn.masked_fill(~valid, float("-inf"))
         elif kv_cache is None or seq_len > 1:
             causal = torch.ones(seq_len, seq_len_kv, dtype=torch.bool, device=q.device).triu(
                 diagonal=1

kempnerforge/model/cross_attention.py

@@ -101,7 +101,12 @@ def forward(
         # the right thing across heads and text Q positions.
         attn_mask = None
         if image_mask is not None:
-            attn_mask = image_mask.view(batch, 1, 1, num_image_tokens)
+            # NaN guard: a sample with no valid image tokens (e.g. an undecodable
+            # clip — all frames padded) would softmax over all -inf -> NaN. Unmask
+            # such rows so softmax stays finite; their text outputs are discarded
+            # (the clip's labels are all -100).
+            safe = image_mask | ~image_mask.any(dim=1, keepdim=True)  # (B, N)
+            attn_mask = safe.view(batch, 1, 1, num_image_tokens)
 
         # Cross-attention: no causal mask on the image axis.
         out = F.scaled_dot_product_attention(q, k, v, attn_mask=attn_mask, is_causal=False)

kempnerforge/model/modality.py

@@ -46,6 +46,9 @@ class ModalityContext:
       error).
     - ``modality_ids`` requires ``prefix_embeds`` or ``inputs_embeds``
       to be set (routing without a residual extension is meaningless).
+    - ``key_padding_mask`` requires ``prefix_embeds`` or ``inputs_embeds``
+      to be set (it is a key-validity mask over the residual sequence; the
+      Cross-Attention arch masks image K/V via ``image_mask`` instead).
 
     ``output_slice`` composes with the ``tokens`` path AND with the
     ``inputs_embeds`` path; it is not constrained intra-context. The
@@ -59,6 +62,7 @@ class ModalityContext:
     image_features: torch.Tensor | None = None
     image_mask: torch.Tensor | None = None
     modality_ids: torch.Tensor | None = None
+    key_padding_mask: torch.Tensor | None = None
 
     def __post_init__(self) -> None:
         residual_routes = sum(
@@ -80,3 +84,12 @@ def __post_init__(self) -> None:
                 "ModalityContext: modality_ids requires prefix_embeds OR "
                 "inputs_embeds to be set (routing without a residual extension is meaningless)"
             )
+        if (
+            self.key_padding_mask is not None
+            and self.prefix_embeds is None
+            and self.inputs_embeds is None
+        ):
+            raise ValueError(
+                "ModalityContext: key_padding_mask requires prefix_embeds OR inputs_embeds "
+                "to be set (it masks the residual sequence; Cross-Attention uses image_mask)"
+            )