cohere: stop degenerate repetition in the greedy decoder

[alexxxcoelho]

cohere: stop degenerate repetition in the greedy decoder

The bug

src/arch/cohere/model.cpp selects each token with a plain argmax over the
logits (three sites: the prompt pass at ~L1063 and both step loops). There is no
repetition penalty, no no_repeat_ngram, and no coverage term. The only exits
are eos_id and the token budget.

That leaves nothing to break a self-reinforcing state: when the most likely
continuation of a phrase is that same phrase, the decoder emits it until the
budget runs out. The result is a transcript that repeats one sentence dozens of
times, returned with TRANSCRIBE_OK and no diagnostic beyond the
output truncated at N tokens warning — which fires because of the loop, not
before it.

Evidence that this is the search, not the weights

Reproduced on 60 problem recordings from a French dictation corpus, decoded
twice with the same audio and the same parameters:

	Q8_0	Q4_K_M
files with an 8-gram repeated ≥5×	1	1
files where Q4 loops more than Q8	—	0
files where Q4 loops less than Q8	—	0

On the worst case the two quantisations loop token for token: 6 repetitions,
43 words, identical output. Quantisation is not the cause.

A related observation: every output truncated at 512 tokens warning in that
corpus landed on a looping file. The cap is a symptom, not the disease.

The fix

A tail-repetition guard in the decode loop, on both the static-graph (GPU) and
dynamic-graph (CPU) paths.

After each token is appended, looping_tail_block() reports the length of a
block that repeats at the end of the generated sequence. A block counts only if
it recurs at least kMinLoopRepeat (3) times and spans at least
kMinLoopTokens (12) tokens in total — so a 1-token block needs 12 repeats
while a 4-token block needs 3. That floor is what keeps an emphatic
"no no no" from being read as a loop.

When a loop is found, trim_looping_tail() drops every copy of the block but
the first, the decoder logs a warning, and decoding stops as if EOS had been
reached. Text preceding the loop is kept — it is usually correct; only the
runaway tail is discarded.

Only the tail is examined, so a deliberate repetition earlier in an utterance
survives untouched.

Results

On the same 60 problem files:

	before	after
files with an 8-gram repeated ≥5×	1	0
maximum repetition observed	6	2
files whose text changed at all	—	4 of 60

56 of 60 files are byte-identical, so the guard is not rewriting healthy output.

Independently verified on a second corpus of 7,724 speech units (28.9 h,
French meeting recordings): 36 loops stopped at the source, and a mechanical
scan of the outputs found no residual loop.

Notes

• No new dependencies; <algorithm> is added for std::max.
• The guard is unconditional. If you would rather gate it, the natural place is
  a field on transcribe_run_params, and I am happy to rework it that way.
• The same greedy-argmax pattern appears in the canary and canary_qwen
  decoders, which carry the same output truncated at %d tokens warning. I have
  not reproduced a loop there, so I have left them alone rather than change code
  I could not test.

[cjpais]

Thanks I'll take a look and likely pull in, I think the reference doesn't do this but this should be helpful in the general case so we should