[NNX] Delete Linen (12.1a): collapse dispatch in the core train loop and maxtext_utils

[ecnal-cienet]

…ate setup and train loop

The pure_nnx defaults are true, so the Linen branches in the pre-train state and train-loop path are dead. Collapse them:

• train_utils.setup_train_loop: always build the abstract NNX model and a TrainStateNNX init_state_fn; drop the Linen model/TrainState branch and the Linen arms of the DiLoCo sharding and debug_sharding blocks.
• maxtext_utils: get_functional_train_with_signature and get_functional_eval_with_signature drop the trailing rng in_sharding; load_compiled drops the example rng; get_abstract_state delegates to get_abstract_state_nnx.

setup_initial_state is deliberately left alone. Its Linen branch is entangled with the checkpoint restore overlay, and the orbax v1 migration is touching that code; it is collapsed in a later change.

Tests follow the same narrowing: the Linen-only cases in maxtext_utils_test, state_dtypes_test and the sharding_compare_test Linen-golden driver go away. test_deepseek4 is skipped rather than pinned to Linen, since nnx_decoders.py has no deepseek4 decoder_block branch yet.

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Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/utils/train_utils.py	75.00%	3 Missing and 1 partial ⚠️

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