imatrix: fix division by zero

[krampenschiesser]

Overview

Viewing the imatrix statistics via ./llama-imatrix --in-file imatrix.gguf --show-statistics does not work in recent llama versions.
It segfaults when opening any imatrix files (unsloth, mraderbacher, local) because the context length in m_params is 0 and then hard-coded in the example.
This is a super basic fix to see the statistics again by using the hardcoded context length from the example itself when its stored as 0.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: NO ai was used

[CISC]

It can be much more basic than this (while also supporting legacy imatrix.dat), essentially m_last_chunk is not used anyway when using --show-statistics, so just force params.n_ctx = 512 (hardcode it, as this is legacy default, no need for global) here:

llama.cpp/tools/imatrix/imatrix.cpp

Line 1228 in 2d46b0f

if (!show_statistics(params)) {

[krampenschiesser]

So i can completely remove the m_last_chunk calculation from the imatrix loading?

[CISC]

So i can completely remove the m_last_chunk calculation from the imatrix loading?

No, it's needed when not using --show-statistics, the point is you can just force the legacy default then so it won't break; a simple one-liner.