fix: prevent core dump in CPU backend if using FP8 (e4m3) tensors

[akleine]

Summary

This patch should solve #2139

Related Issue / Discussion

It seems the result of ggml_backend_supports_op(ctx->backend, fp8_matmul) is wrong for CPU backend ,
so I have added another check here avoiding this FP8 op in CPU backend
Maybe it is a GGML bug ? Anyway I think sd.cpp should be independent of bug fixing there.
Maybe I am wrong ?

Additional Information

Checklist

• I have read and confirmed this PR follows the contribution guidelines.

Summary by CodeRabbit

• Bug Fixes
  • Improved CPU handling for FP8 operations by using a BF16 fallback when direct support is unavailable.

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• src/model/common/ggml_block.hpp

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📝 Walkthrough

Linear::forward now uses FP8 matrix multiplication only when the backend supports the operation and is not a CPU backend. CPU backends use the existing BF16 weight-cast fallback.

Changes: CPU FP8 Matmul Fallback

Layer / File(s)	Summary
CPU backend support check src/model/common/ggml_block.hpp	Linear::forward checks the backend type before using FP8 matrix multiplication. CPU backends take the existing BF16 weight-cast fallback.

Priority: ➖ Normal

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix

Suggested reviewers: leejet

Merge Risk: ⚪ Minimal · up to ec5fc

CPU backends that report FP8 support now use the BF16 fallback, while supported non-CPU backends retain FP8 matmul. No concrete merge-blocking issue was established.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: preventing a CPU backend core dump when FP8 E4M3 tensors are used.
Description check		The description explains the change, identifies issue #2139, provides implementation context, and includes the completed checklist. The Additional Information section is empty, but this is non-critica…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/model/common/ggml_block.hpp:
- Line 225: Update the `supports_fp8_matmul` condition to use
`sd_backend_is_cpu` instead of checking whether the backend name contains “cpu,”
so only CPU backends disable FP8 matmul and RPC GPU backends remain eligible.

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• src/model/common/ggml_block.hpp

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