model : make minicpm embedding_scale, residual_scale and logit_scale optional with...

model : make minicpm embedding_scale, residual_scale and logit_scale optional with legacy defaults (#16273)

* minicpm: make GGUF scaling keys optional with legacy defaults

Older MiniCPM GGUFs do not include the scaling metadata keys (minicpm.embedding_scale, minicpm.residual_scale, minicpm.logit_scale). The loader currently treats these as required, so quantization fails with:

    key not found in model: minicpm.embedding_scale

This change restores backward compatibility by treating these keys as optional in the loader and using the older MiniCPM scaling values:

    embedding_scale = 12.0f
    residual_scale  = 1.4f / sqrt(n_layer)
    logit_scale     = 256.0f / n_embd

When the GGUF provides the keys, their values override the defaults; otherwise the legacy defaults are used. Newer GGUFs that already include these keys are unaffected.

Fixes: #16192
Signed-off-by: Vinkal Chudgar <redacted>
* Update src/llama-model.cpp

Committed as suggested. Thanks!

Co-authored-by: Sigbjørn Skjæret <redacted>
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Signed-off-by: Vinkal Chudgar <redacted>
Co-authored-by: Sigbjørn Skjæret <redacted>