Fix Gemma remat static argument for dropout

[primorLee]

Summary

• correct the lifted nn.remat static argument index for Gemma blocks
• add a regression test that initializes the scanned Gemma module with dropout enabled

Why

Flax Linen's lifted checkpoint transform interprets static_argnums against the original module method signature, including self, and then shifts the indices internally. Block.__call__ therefore needs index 6 for deterministic.

With index 5, adarms_cond is selected instead and deterministic remains traced. Enabling Gemma dropout then reaches Flax's Python boolean branch with a traced value and raises TracerBoolConversionError during model initialization.

The regression test uses the real gemma.Module remat + scan + dropout path with a one-layer dummy config and a reduced vocabulary for speed.

Fixes #987

Testing

• pytest -q src/openpi/models/gemma_test.py (1 passed)
• pre-commit run --files src/openpi/models/gemma.py src/openpi/models/gemma_test.py