[fix]: magi-human latent prep: honor batch.num_frames

The latent preparation stage was deriving num_frames from
batch.num_seconds * fps + 1 unconditionally and ignoring
batch.num_frames. num_seconds was never set anywhere (no plumbing in
SamplingParam or ForwardBatch), so every generation defaulted to 4
seconds = 101 frames, regardless of what the caller passed via
SamplingParam.num_frames.

Now we prefer batch.num_frames when it's a sensible video length (>1)
and fall back to the seconds-based derivation when the caller explicitly
passes num_seconds. Backward-compatible with the production preset
(num_frames=101 == 4*25+1). Fixes the SSIM test budget — it was running
at 101 frames despite asking for 26.

All 8 runnable magi-human parity tests still pass.

fastvideo/pipelines/basic/magi_human/stages/latent_preparation.py

@@ -129,9 +129,20 @@ class MagiHumanLatentPreparationStage(PipelineStage):
 
     def forward(self, batch: ForwardBatch, fastvideo_args: FastVideoArgs) -> ForwardBatch:
         device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
-        seconds = int(getattr(batch, "num_seconds", None) or 4)
         fps = self.fps
-        num_frames = seconds * fps + 1
+        # Prefer the caller-provided `batch.num_frames` (the standard
+        # SamplingParam knob — production preset and SSIM tests both set
+        # it). Fall back to `batch.num_seconds * fps + 1` when num_frames
+        # is unset or the image-default sentinel (1). This matches
+        # upstream MagiDataProxy.process_input which derives `num_frames
+        # = seconds * fps + 1` and rejects values that don't satisfy
+        # `(num_frames - 1) % vae_temporal_stride == 0`.
+        requested_num_frames = int(getattr(batch, "num_frames", None) or 0)
+        if requested_num_frames > 1:
+            num_frames = requested_num_frames
+        else:
+            seconds = int(getattr(batch, "num_seconds", None) or 4)
+            num_frames = seconds * fps + 1
         latent_T = (num_frames - 1) // 4 + 1
 
         # Match upstream pipeline.py:61-64 + video_generate.py:254-261: