TensorFlow RaggedTensor Support (batch-level)

### Feature request

Hi,

Currently datasets does not support RaggedTensor output on batch-level.
When building a Object Detection Dataset (with TensorFlow) I need to enable RaggedTensors as that's how BBoxes & classes are expected from the Keras Model POV.

Currently there's a error thrown saying that "Nested Data is not supported".

It'd be very helpful if this was fixed! :)

### Motivation

Enabling Object Detection pipelines for TensorFlow.

### Your contribution

With guidance I'd happily help making the PR.

The current implementation with DataCollator and later enforcing `np.array` is the problematic part (at the end of `np_get_batch` in `tf_utils.py`). As `numpy` don't support "Raggednes"