Fix DatasetDict examples in the NumPy and JAX guides

[shaikn6]

The DatasetDict examples in the NumPy and JAX guides call DatasetDict.from_dict(data), but DatasetDict has no from_dict (only Dataset does), so the snippet fails:

AttributeError: type object 'DatasetDict' has no attribute 'from_dict'

This builds the DatasetDict from Dataset.from_dict(...) per split instead, keeping the same data dict and the lines after it.

Checked against main (9e7496a): the old snippet raises the error above, and both updated snippets run and return the train rows shown in the docs. I left the JAX output as is; current JAX prints Array(...) where the page shows DeviceArray(...), which is the case for the other JAX snippets on that page too.