Fix garbled output with REPACK at high thread counts (#16956)

Fix garbled output with REPACK at high thread counts (#16956)

* Fix garbled output with REPACK at high thread counts

Fixed a race condition in the REPACK matrix multiplication code that caused garbled output when using 26+ threads (model-dependent threshold). The issue occurred because with high thread counts, the code forced chunk count to equal thread count, creating many small chunks. After aligning these chunks to NB_COLS boundaries, adjacent chunks could overlap, causing data corruption and race conditions. The fix enforces minimum chunk sizes based on NB_COLS and caps maximum chunk count to prevent creating too many tiny chunks, ensuring proper alignment without overlaps.

* Update ggml/src/ggml-cpu/repack.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update ggml/src/ggml-cpu/repack.cpp

Co-authored-by: Georgi Gerganov <redacted>
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Co-authored-by: Georgi Gerganov <redacted>