improve documentation

lerobot/common/datasets/lerobot_dataset.py

@@ -29,6 +29,24 @@ def __init__(
         delta_timestamps: dict[list[float]] | None = None,
         n_end_keyframes_dropped: int = 0,
     ):
+        """
+        Args:
+            delta_timestamps: A dictionary mapping lists of relative times (Δt) to data keys. When a frame is
+                sampled from the underlying dataset, we treat it as a "keyframe" and load multiple frames
+                according to the list of Δt's. For example {"action": [-0.05, 0, 0.05]} indicates
+                that we want to load the current keyframe's action, as well as one from 50 ms ago, and one
+                50 ms into the future. The action key then contains a (3, action_dim) tensor (whereas without
+                `delta_timestamps` there would just be a (action_dim,) tensor. When the Δt's demand that
+                frames outside of an episode boundary are retrieved, a copy padding strategy is used. See
+                `load_previous_and_future_frames` for more details.
+            n_end_keyframes_dropped: Don't sample the last n items in each episode. This option is handy when
+                used in combination with `delta_timestamps` when, for example, the Δt's demand multiple future
+                frames, but we want to avoid introducing too much copy padding into the data distribution.
+                For example if `delta_timestamps = {"action": [0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30]}`
+                and we sample the last frame in the episode, we would end up padding with 6 frames worth of
+                copies. Instead, we might want no padding (in which case we need n=6), or we might be okay
+                with up to 2 frames of padding (in which case we need n=4).
+        """
         super().__init__()
         self.repo_id = repo_id
         self.version = version