Validate interior padding length in Nx.pad

[polvalente]

The documentation says negative values in interior padding is not allowed, and logic follows that this doesn't make sense.
We should be validating this in Nx.Shape.pad.

[josevalim]

The issue is that in EXLA we can’t really validate values and shape doesnt have values either. We may be able to raise in the binary backend but that’s itz

[polvalente]

To be clear, I'm talking about the {before, after, interior} values in the padding config. These are known at compile time