spec : add DFlash support (#22105)

* spec: add DFlash v2 support

* dflash: support sliding window attention per layer_types

* docs: add dflash section

---------

Co-authored-by: Kashif Rasul <kashif.rasul@gmail.com>

docs/speculative.md

@@ -52,6 +52,32 @@ Supported EAGLE-3 draft models include:
 
 For the full and up-to-date list of supported models, see #18039.
 
+### DFlash (`draft-dflash`)
+
+DFlash produces an entire block of draft tokens in a single forward pass (block diffusion) and
+injects the target model's hidden states into the draft model's attention, instead of drafting one
+token at a time. This keeps the draft model small while making drafting GPU-friendly. Unlike EAGLE-3
+(a single-layer autoregressive draft), the DFlash draft uses several transformer layers but emits a
+whole block per draft step.
+
+The draft is a small block-diffusion model trained for a specific target (for example
+`z-lab/Qwen3-4B-DFlash` for `Qwen/Qwen3-4B`). Convert it with `--target-model-dir` so it inherits the
+target's tokenizer and token embeddings:
+
+```bash
+python convert_hf_to_gguf.py z-lab/Qwen3-4B-DFlash \
+    --target-model-dir Qwen/Qwen3-4B --outtype bf16 --outfile Qwen3-4B-DFlash.gguf
+
+llama-server -m Qwen3-4B.gguf -md Qwen3-4B-DFlash.gguf \
+    --spec-type draft-dflash --spec-draft-n-max 15 -fa on --jinja
+```
+
+`--spec-draft-n-max` is clamped to the draft model's trained block size.
+
+See:
+
+- #22105
+
 ### n-gram Cache (`ngram-cache`)
 
 An n-gram is a sequence of n tokens. The n-gram cache implementation maintains statistics about short n-gram sequences.
@@ -147,7 +173,7 @@ If a draft model is combined with a draftless decoding the draftless decoding ha
 ### General Speculative Parameters
 
 ```
---spec-type [none|draft-simple|draft-eagle3|draft-mtp|ngram-cache|ngram-simple|ngram-map-k|ngram-map-k4v|ngram-mod]
+--spec-type [none|draft-simple|draft-eagle3|draft-dflash|draft-mtp|ngram-cache|ngram-simple|ngram-map-k|ngram-map-k4v|ngram-mod]
                                         comma-separated list of types of speculative decoding to use
                                         (default: none)
                                         (env: LLAMA_ARG_SPEC_TYPE)
@@ -287,6 +313,7 @@ Specifies a comma-separated list of speculative decoding types to use.
 | `none` | No speculative decoding (default) |
 | `draft-simple` | Use a simple draft model for speculation |
 | `draft-eagle3` | Use an EAGLE-3 draft model that reads the target's hidden states |
+| `draft-dflash` | Use a DFlash block-diffusion draft model that emits a block per step |
 | `draft-mtp` | Use Multi Token Prediction (MTP) heads from the main model |
 | `ngram-cache` | Use n-gram cache lookup |
 | `ngram-simple` | Use simple n-gram pattern matching |