fix: decoders and pipeline parity gaps of linen to nnx migrations

[mesakhcienet]

Description

Closes the remaining behavioral gaps between the pure-NNX decoder/pipeline path and the Linen reference.
The NNX path now reproduces Linen for DeepSeek-V4, per-stage remat, and the pipeline's non-trainable / repeat-level-remat handlning.

What's included

DeepSeek-V4 NNX decoder port

• Registered DEEPSEEK4 in NNXDecoder.get_decoder_layer(was missing → ValueError at construction) and added full decoder-level handling: norm dispatch (RMSNorm), scanned + non-scanned init, _apply_deepseek4_scanned_blocks (prefix first_num_hash_layers unroll + paired HCA/CSA scan), global layer_idx, and decoder_input_tokens threading — matching Linen _apply_deepseek4_scanned_blocks.

Per-stage pipeline remat parity (set_remat_policy_on_layers_per_stage)

• The flag was a no-op in the NNX pipeline after the Linen→NNX migration. Restored per-stage remat (jax.checkpoint) + params-only host-offload in NNXSequentialPipelineStage / NNXScannedPipelineStage, wired from both stage builders, incl. num_layers_per_pipeline_stage == 1.
• Fix: decoupled "apply remat" from the policy value. remat_policy='full' resolves to None (== full remat, as Linen nn.remat(policy=None)); the old if policy is not None gate silently dropped remat for the default 'full' policy. Now gated on the flag via an explicit apply_remat argument.

Pipeline Linen→NNX migration parity (pipeline.py)

• non_trainable collection: the migration asserted the iteration-scan catch-all was RngState-only, crashing any pipelined model with a non-trainable variable (e.g. the DeepSeek-V4 hash-routing table). Non-circular now broadcasts non_trainable as a loop-invariant constant (4-way state split); circular carries it via
  carry_state.
• circular repeat-level remat: made unconditional to match the Linen reference (whose flag-check was dead code, always rematting). Default flag path unchanged; only flag=False configs regain the dropped rematerialization.

Unit Tests

• tests/unit/nnx_decoders_test.py (+ DeepSeek-V4 construct/forward/scan parity, pipeline-stage forward + remat transparency, per-stage-remat-applied guards, layer_map registration guard).
• tests/unit/nnx_pipeline_test.py (new): coverage for NNXPipeline / NNXCircularPipeline — non-circular + circular forward, non_trainable partitioning, repeat-remat output transparency.

Tests

Sheet combination of set_remat_policy_on_layers_per_stage flag.

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
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• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

❌ Patch coverage is 82.50000% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/layers/nnx_decoders.py	89.06%	3 Missing and 4 partials ⚠️
src/maxtext/layers/decoders.py	0.00%	5 Missing ⚠️
src/maxtext/layers/pipeline.py	81.81%	2 Missing ⚠️

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[mesakhcienet]

remove get_layer_to_pipeline dead code (unused anymore)