pyarrow.lib.ArrowNotImplementedError: MakeBuilder: cannot construct builder for type extension<arrow.py_extension_type>
Yea unfortunately..
That's a current limitation with Arrow ExtensionTypes that can't be used in the default Arrow Array objects.
We already have an ExtensionArray that allows us to use them as column types but not for subtypes.
Maybe we can extend it, I haven't experimented with that yet
I set up a new dataset, with a sequence of arrays (really, I want to have an array of (None, 137, 2), and the first dimension is dynamic) 

```python
    def _info(self):
        return datasets.DatasetInfo(
            description=_DESCRIPTION,
            # This defines the different columns of the dataset and their types
            features=datasets.Features(
                {
                    "pose": datasets.features.Sequence(datasets.features.Array2D(shape=(137, 2), dtype="float32"))
                }
            ),
            homepage=_HOMEPAGE,
            citation=_CITATION,
        )
    def _generate_examples(self):
        """ Yields examples. """

        yield 1, {
            "pose": [np.zeros(shape=(137, 2), dtype=np.float32)]
        }
```

But this doesn't work -
> pyarrow.lib.ArrowNotImplementedError: MakeBuilder: cannot construct builder for type extension<arrow.py_extension_type>
pyarrow.lib.ArrowNotImplementedError: MakeBuilder: cannot construct builder for type extension<arrow.py_extension_type> 
 I set up a new dataset, with a sequence of arrays (really, I want to have an array of (None, 137, 2), and the first dimension is dynamic) 

```python
    def _info(self):
        return datasets.DatasetInfo(
            description=_DESCRIPTION,
            # This defines the different columns of the dataset and their types
            features=datasets.Features(
                {
                    "pose": datasets.features.Sequence(datasets.features.Array2D(shape=(137, 2), dtype="float32"))
                }
            ),
            homepage=_HOMEPAGE,
            citation=_CITATION,
        )
    def _generate_examples(self):
        """ Yields examples. """

        yield 1, {
            "pose": [np.zeros(shape=(137, 2), dtype=np.float32)]
        }
```

But this doesn't work -
> pyarrow.lib.ArrowNotImplementedError: MakeBuilder: cannot construct builder for type extension<arrow.py_extension_type> 
 Yea unfortunately..
That's a current limitation with Arrow ExtensionTypes that can't be used in the default Arrow Array objects.
We already have an ExtensionArray that allows us to use them as column types but not for subtypes.
Maybe we can extend it, I haven't experimented with that yet