sampling: GPU argmax fast path for greedy decode

sample() at temperature 0 copied the full [seq, 201088] BF16 logits
to the host and scanned them every token (~1 ms/token). Use the
Phase 15 argmax kernel (block reduction + 4-byte D2H) when logits are
BF16 on GPU; bench-gpt-oss's greedy sampler likewise. Exact-tie
logits may break differently than the host scan — greedy trajectories
can legitimately diverge at a tie token (GSM8K unchanged).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/xserv-model/src/sampling.rs

@@ -19,6 +19,18 @@ impl Default for SamplingParams {
 /// Uses the last position's logits. Handles both F32 and BF16 dtypes.
 pub fn sample(logits: &Tensor, params: &SamplingParams) -> u32 {
     assert_eq!(logits.ndim(), 2);
+    // Greedy fast path: GPU argmax + 4-byte D2H instead of copying the whole
+    // [seq, vocab] logits to the host and scanning it (~201k bf16/token).
+    // NaN logits lose every `>` comparison in the kernel, matching the
+    // NaN-safe host argmax below.
+    if params.temperature == 0.0
+        && logits.dtype() == DType::BF16
+        && matches!(logits.device(), Device::Cuda(_))
+        && logits.is_contiguous()
+    {
+        let ids = xserv_kernels::argmax_bf16_to_host(logits);
+        return *ids.last().unwrap();
+    }
     let vocab_size = logits.shape()[1];
     let seq_len = logits.shape()[0];
     let logits_cpu = logits.to_device(Device::Cpu);