Replace _graphdef with _ for unused variables in NNX tutorials

[Copilot]

Replace prefixed unused variable _graphdef with the standard Python convention _ for intentionally unused return values in tutorial examples.

Changes

• tutorials/v1/device/08_initialize_nnx_on_device.py: Replace _graphdef, state = nnx.split(model) with _, state = nnx.split(model) (2 instances)
• tutorials/v1/device/09_split_learning_vertical.py: Replace _graphdef, state = nnx.split(model) with _, state = nnx.split(model) (3 instances in model initialization functions)

# Before
_graphdef, state = nnx.split(model)

# After
_, state = nnx.split(model)

The graphdef is not needed in these contexts as models are reconstructed from their class definitions when deserializing state dicts.

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[da-niao-dan]

lgtm