lfm2 : make LFM2.5-VL-3B work with a DSpark draft model

[tugot17]

This makes LFM2.5-VL-3B work with a DSpark draft model.

The lfm2 graph lost its t_layer_inp capture in the models/ split (still there at 38a5b42, and qwen3.cpp kept its copy). Without it the server asserts on the first request whenever a drafter is attached:

llama-graph.cpp:1352: GGML_ASSERT(t_layer_inp[il] != nullptr && "layer input tensor is null")

One line to put it back.

Testing:

hf download LiquidAI/LFM2.5-VL-3B-GGUF LFM2.5-VL-3B-F16.gguf mmproj-LFM2.5-VL-3B-F16.gguf --local-dir models
hf download tugot17/mango-gguf mango-draft-F16.gguf --local-dir models

./build/bin/llama-server -m models/LFM2.5-VL-3B-F16.gguf \
  --mmproj models/mmproj-LFM2.5-VL-3B-F16.gguf \
  -md models/mango-draft-F16.gguf \
  --spec-type draft-dspark --spec-draft-n-max 8 --spec-draft-n-min 0 \
  -fa on -ngl 99 -c 8192

Target and mmproj are public, the draft sidecar is gated on my HF, ping me for access.

Accept length on a 20-sample MMSpec probe: 3.86 (at 38a5b42), our internal fork gets ~4.1 on the same samples. Greedy output is identical with and without the drafter.

Things I hit on the way that this PR does not fix:

• accept length dropped ~30% for every DSpark draft between 38a5b42 and current master (3.86 -> 2.85, same probe). needs a bisect, my guess is spec: support speculators-format checkpoints for DSpark #26275
• the Lfm2DSparkDraftModel converter path is gone from master, I converted the sidecar with an older tree. btw the old converter applied the q/k rope permute twice for rope_is_neox_style=false checkpoints, which is why all our earlier VL sidecars measured ~1.7. it has to be applied once
• convert_hf_to_gguf --target-model-dir doesn't accept a VL target dir, Lfm2VlForConditionalGeneration is only in the mmproj map

[ggml-gh-bot]

Hi @tugot17, thanks for your contribution!

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[ngxson]

cc @tdakhran if you have have a look

[tdakhran]

This line was already added in https://github.com/ggml-org/llama.cpp/pull/27383/changes#diff-a594bdf207682148894799362abd56072082a07d86986283752f47172ae9babaR252-R253 , why did it disappear?

[tdakhran]

The PR is based on the commit before DSpark support for LFM2 was merged https://github.com/ggml-org/llama.cpp/pull/27383/changes#diff-a594bdf207682148894799362abd56072082a07d86986283752f47172ae9babaR252-R253

please cancel.

cc: @CISC @ngxson