stack vectors in pytorch and tensorflow
When the format of a dataset is set to pytorch or tensorflow, and if the dataset has vectors in it, they were not stacked together as tensors when calling `dataset[i:i + batch_size][column]` or `dataset[column]`.

I added support for stacked tensors for both pytorch and tensorflow.
For ragged tensors, they are stacked only for tensorflow as pytorch doesn't support ragged tensors.
