Add direct penzai.treescope support for NNX objects.
This change implements the `__penzai_repr__` protocol on most NNX
objects, making it possible to directly visualize them using the
standard `penzai.treescope` configuration without an extra conversion
step. Modules, GraphDefs, and States are all visualizable.
The `nnx.display` function is no longer needed if Penzai is installed,
since `pz.ts.basic_interactive_setup()` followed by
`IPython.display.display` or `pz.show` (or just returning an object
from an IPython cell) is now sufficient to visualize NNX objects.
Also fixes GraphDef __repr__ to use "leaves" instead of "variables".
PiperOrigin-RevId: 638280968
