Invalid Tensor Index 'ragged_flat_values' when using Sequence as top-level feature

Short description
I'm trying to add a new dataset. Serialization succeeded, but when loading the data, the error below occurs.

TypeError: Only integers, slices (`:`), ellipsis (`...`), tf.newaxis (`None`) and scalar tf.int32/tf.int64 tensors are valid indices, got 'ragged_flat_values'  

Detailed stacktrace below, the error happens in the deserialization.

Environment information

• Operating System: ubuntu 18.04
• Python version: 3.6
• tensorflow-datasets/tfds-nightly version: tfds-nightly
• tensorflow/tensorflow-gpu/tf-nightly/tf-nightly-gpu version: tested on tensorflow 2.2.0 and 2.3.0

Reproduction instructions

https://github.com/hermannsblum/tf_datasets/blob/3dpw/tensorflow_datasets/human_pose/pose_3dpw.py

Link to logs

stack trace

Expected behavior
The dataset to load, or an error at dataset building.

Additional context
Add any other context about the problem here.

Start with the reproduction in tensorflow_datasets/human_pose/pose_3dpw.py and review the linked stack trace to trace deserialization of a top-level Sequence feature. Confirm the behavior with TensorFlow 2.2.0 or 2.3.0; done means the dataset loads successfully or rejects the configuration during dataset building.