model : add label for LiquidAI LFM2-2.6B model (#16204)

model : add label for LiquidAI LFM2-2.6B model (#16204)

* model : add label for LiquidAI LFM2-2.6B model

HF link: [LiquidAI/LFM2-2.6B](https://huggingface.co/LiquidAI/LFM2-2.6B).

Support for GGUF conversion and inference is added in #14620.

However, due to similar `n_embd`, it identifies as a 1.2B model.
Fix the label by using `n_ff` to identify the model instead.

Output of `llama-bench`:
```
| model                          |       size |     params | backend    | threads |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | ------: | --------------: | -------------------: |
| lfm2 1.2B F16                  |   2.18 GiB |     1.17 B | CPU        |      10 |           pp512 |        223.97 ± 5.32 |
| lfm2 2.6B F16                  |   4.79 GiB |     2.57 B | CPU        |      10 |           pp512 |         92.53 ± 4.14 |
| lfm2 350M F16                  | 676.25 MiB |   354.48 M | CPU        |      10 |           pp512 |       725.52 ± 11.70 |
| lfm2 700M F16                  |   1.38 GiB |   742.49 M | CPU        |      10 |           pp512 |       336.22 ± 12.93 |
```

* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
---------

Co-authored-by: Sigbjørn Skjæret <redacted>