Implement no_repeat_ngram_size for CPU search

[mustjab]

The no_repeat_ngram_size search option was parsed from genai_config.json and settable via set_search_options / OgaGeneratorParamsSetSearchNumber, but no logits processor enforced it, so it had no effect. Greedy decoding could emit unbounded repeated n-grams (e.g. Marian translation looping to max_length on degenerate input).

Add a CPU logits processor that bans any token which would complete an already-seen n-gram of the configured size, matching HuggingFace no_repeat_ngram_size semantics.

Changes:

• search.h: virtual ApplyNoRepeatNgram no-op default (CUDA/beam unaffected), Search_Cpu override.
• search.cpp: ApplyNoRepeatNgram implementation.
• generators.cpp, engine/request.cpp: invoke after ApplyRepetitionPenalty.
• config.h: document the field.
• sampling_tests.cpp: NoRepeatNgramCorrectnessCpu unit test.

[Copilot]

Pull request overview

This PR makes the existing no_repeat_ngram_size search option actually take effect for CPU decoding by adding a CPU-side logits processor that bans tokens which would complete a previously seen n-gram (matching HuggingFace semantics). This closes a functional gap where the option was parsed and settable but previously had no impact on generation behavior.

Changes:

• Add a Search::ApplyNoRepeatNgram() virtual hook (default no-op) and implement the CPU override to ban tokens that would repeat an n-gram.
• Invoke ApplyNoRepeatNgram() during per-token scoring in both the direct generator path and the engine request path.
• Add a CPU correctness unit test that forces greedy decoding to choose a fallback token when the highest-scoring token would repeat an n-gram.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/sampling_tests.cpp	Adds a CPU unit test validating greedy decoding respects no_repeat_ngram_size.
src/search.h	Introduces ApplyNoRepeatNgram() (default no-op) and declares CPU override.
src/search.cpp	Implements CPU n-gram blocking by banning tokens that would repeat a seen n-gram.
src/generators.cpp	Calls ApplyNoRepeatNgram() in the non-engine generation loop after repetition penalty.
src/engine/request.cpp	Calls ApplyNoRepeatNgram() in the engine request generation loop after repetition penalty.
src/config.h	Updates documentation comment for no_repeat_ngram_size to reflect actual behavior.