Improve dataset examples (#82)

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

examples/1_load_hugging_face_dataset.py

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+"""
+This script demonstrates the visualization of various robotic datasets from Hugging Face hub.
+It covers the steps from loading the datasets, filtering specific episodes, and converting the frame data to MP4 videos.
+Importantly, the dataset format is agnostic to any deep learning library and doesn't require using `lerobot` functions.
+It is compatible with pytorch, jax, numpy, etc.
+
+As an example, this script saves frames of episode number 5 of the PushT dataset to a mp4 video and saves the result here:
+`outputs/examples/1_visualize_hugging_face_datasets/episode_5.mp4`
+
+This script supports several Hugging Face datasets, among which:
+1. [Pusht](https://huggingface.co/datasets/lerobot/pusht)
+2. [Xarm Lift Medium](https://huggingface.co/datasets/lerobot/xarm_lift_medium)
+3. [Aloha Sim Insertion Human](https://huggingface.co/datasets/lerobot/aloha_sim_insertion_human)
+4. [Aloha Sim Insertion Scripted](https://huggingface.co/datasets/lerobot/aloha_sim_insertion_scripted)
+5. [Aloha Sim Transfer Cube Human](https://huggingface.co/datasets/lerobot/aloha_sim_transfer_cube_human)
+6. [Aloha Sim Transfer Cube Scripted](https://huggingface.co/datasets/lerobot/aloha_sim_transfer_cube_scripted)
+
+To try a different Hugging Face dataset, you can replace this line:
+```python
+hf_dataset, fps = load_dataset("lerobot/pusht", split="train"), 10
+```
+by one of these:
+```python
+hf_dataset, fps = load_dataset("lerobot/xarm_lift_medium", split="train"), 15
+hf_dataset, fps = load_dataset("lerobot/aloha_sim_insertion_human", split="train"), 50
+hf_dataset, fps = load_dataset("lerobot/aloha_sim_insertion_scripted", split="train"), 50
+hf_dataset, fps = load_dataset("lerobot/aloha_sim_transfer_cube_human", split="train"), 50
+hf_dataset, fps = load_dataset("lerobot/aloha_sim_transfer_cube_scripted", split="train"), 50
+```
+"""
+
+from pathlib import Path
+
+import imageio
+from datasets import load_dataset
+
+# TODO(rcadene): list available datasets on lerobot page using `datasets`
+
+# download/load hugging face dataset in pyarrow format
+hf_dataset, fps = load_dataset("lerobot/pusht", split="train"), 10
+
+# display name of dataset and its features
+print(f"{hf_dataset=}")
+print(f"{hf_dataset.features=}")
+
+# display useful statistics about frames and episodes, which are sequences of frames from the same video
+print(f"number of frames: {len(hf_dataset)=}")
+print(f"number of episodes: {len(hf_dataset.unique('episode_id'))=}")
+print(f"average number of frames per episode: {len(hf_dataset) / len(hf_dataset.unique('episode_id')):.3f}")
+
+# select the frames belonging to episode number 5
+hf_dataset = hf_dataset.filter(lambda frame: frame["episode_id"] == 5)
+
+# load all frames of episode 5 in RAM in PIL format
+frames = hf_dataset["observation.image"]
+
+# save episode frames to a mp4 video
+Path("outputs/examples/1_load_hugging_face_dataset").mkdir(parents=True, exist_ok=True)
+imageio.mimsave("outputs/examples/1_load_hugging_face_dataset/episode_5.mp4", frames, fps=fps)