fix(smolvla): correct loss normalization for padded actions (#3434)

Apply the same per-scalar-mean fix to SmolVLA that #3377 landed for
ACT / Diffusion / MultiTaskDiT. The pre-patch form applies the
`action_is_pad` mask to zero out padded timesteps, then calls `.mean()`
(or `.mean(dim=(1, 2))`). Because `.mean()` divides by the total number
of elements including the zeroed padding, the loss is diluted by the
padding fraction.

Fixed by normalizing only over valid (non-padded) scalar entries:

    num_valid = ((~actions_is_pad).sum(...) * losses.shape[-1]).clamp_min(1)
    loss = losses.sum(...) / num_valid

`clamp_min(1)` preserves the all-padded-batch edge case (0/1 = 0). Both
reduction paths are updated. Behavior when `action_is_pad` is missing is
unchanged (`losses.mean()`).

Empirical A/B on aloha_sim_transfer_cube_human (chunk_size=40, batch=2,
30 steps, fixed seed, GB200) shows `loss_A / loss_B = 0.9672 (±0.088)` —
same direction and magnitude as PR #3377's `loss_A / loss_C ≈ 0.96` for
ACT. Heavier-padding recipes will see a larger gap.

Refs: #3353 (original report for ACT), #3377 (fix for the other three
policies).

src/lerobot/policies/smolvla/modeling_smolvla.py

@@ -394,13 +394,21 @@ class SmolVLAPolicy(PreTrainedPolicy):
         loss_dict["losses_after_rm_padding"] = losses.clone().mean().item()
 
         if reduction == "none":
-            # Return per-sample losses (B,) by averaging over time and action dims
-            per_sample_loss = losses.mean(dim=(1, 2))
+            # Return per-sample losses (B,) by averaging over valid (time, action) entries
+            if actions_is_pad is None:
+                per_sample_loss = losses.mean(dim=(1, 2))
+            else:
+                num_valid = ((~actions_is_pad).sum(dim=1) * losses.shape[-1]).clamp_min(1)
+                per_sample_loss = losses.sum(dim=(1, 2)) / num_valid
             loss_dict["loss"] = per_sample_loss.mean().item()
             return per_sample_loss, loss_dict
         else:
-            # Default: return scalar mean loss
-            loss = losses.mean()
+            # Default: return scalar mean loss over valid (time, action) entries
+            if actions_is_pad is None:
+                loss = losses.mean()
+            else:
+                num_valid = ((~actions_is_pad).sum() * losses.shape[-1]).clamp_min(1)
+                loss = losses.sum() / num_valid
             loss_dict["loss"] = loss.item()
             return loss, loss_dict