* fix(peft): expose finetune_last_n_layers for parity with mlx-lm CLI
mlx-lm's lora CLI defaults `CONFIG_DEFAULTS['num_layers']=16`
(mlx_lm/lora.py:56), so it applies LoRA only to the LAST 16
transformer blocks. PEFT on the CUDA path supports the same via
`layers_to_transform`, but most users don't reach for it.
This commit adds a `finetune_last_n_layers` convenience parameter
to both `FastLlamaModel.get_peft_model` and
`FastBaseModel.get_peft_model` (vision/multi-modal). When set, it
fills `layers_to_transform` automatically with the last N blocks,
mirroring mlx-lm CLI's behavior AND
`unsloth_zoo.mlx.loader.FastMLXModel.get_peft_model`. A single
config value now controls layer-selection consistently across
CUDA, MLX (zoo), and mlx-lm CLI paths.
Default is None (= train all layers, current behavior unchanged).
When set, the value is clamped to [1, total_transformer_layers]
so callers can't accidentally over- or under-select. The total
is read from `config.num_hidden_layers` (or aliases), falling
through to `config.text_config.num_hidden_layers` for VLMs.
Why this matters: with the same fixture/seed, training the last
N layers vs all layers picks a different basin under stochastic
LoRA init. Empirically (n=15 seeds, gemma-3-270m-it single-row
LoRA memorization, MLX path) last-16 hits 67% greedy-decode
pass rate vs all-18 at 47%. The teacher-forced completion loss
is 0 in both — the model memorizes either way; only the first-
token argmax distribution differs. CUDA fp32 shows the same
pattern. Aligning the layer selection puts CUDA + MLX + mlx-lm
all in the same basin family for parity comparisons.
* [pre-commit.ci] auto fixes from pre-commit.com hooks
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* peft: trim verbose finetune_last_n_layers comments
Per code-comment policy: parameter name is self-documenting, the clamp
and range() construction are obvious. Rationale (mlx-lm CLI parity,
empirical pass-rate data) lives in commit 106c1df4's message and the
PR description.
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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
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