add EpisodeAwareSampler (#217)

Co-authored-by: Alexander Soare <alexander.soare159@gmail.com>

lerobot/configs/policy/diffusion.yaml

@@ -44,6 +44,10 @@ training:
     observation.state: "[i / ${fps} for i in range(1 - ${policy.n_obs_steps}, 1)]"
     action: "[i / ${fps} for i in range(1 - ${policy.n_obs_steps}, 1 - ${policy.n_obs_steps} + ${policy.horizon})]"
 
+  # The original implementation doesn't sample frames for the last 7 steps,
+  # which avoids excessive padding and leads to improved training results.
+  drop_n_last_frames: 7  # ${policy.horizon} - ${policy.n_action_steps} - ${policy.n_obs_steps} + 1
+
 eval:
   n_episodes: 50
   batch_size: 50