fix(act): disable VAE during offline inference (#1588)

Prevent VAE inference when running in offline mode. In the lerobot dataset, the presence of the 'action' field incorrectly triggers the VAE inference block. This leads to a RuntimeError due to mismatched tensor dimensions (3 vs 2) when concatenating cls_embed, robot_state_embed, and action_embed—since action_embed lacks the chunk_size dimension. Additionally, this aligns with the original paper, where variational inference is skipped during inference.

src/lerobot/policies/act/modeling_act.py

@@ -420,7 +420,7 @@ class ACT(nn.Module):
             batch_size = batch["observation.environment_state"].shape[0]
 
         # Prepare the latent for input to the transformer encoder.
-        if self.config.use_vae and "action" in batch:
+        if self.config.use_vae and "action" in batch and self.training:
             # Prepare the input to the VAE encoder: [cls, *joint_space_configuration, *action_sequence].
             cls_embed = einops.repeat(
                 self.vae_encoder_cls_embed.weight, "1 d -> b 1 d", b=batch_size