[Bugfix] Initialize draft CUDA-graph keys for the native draft_model proposer

[avalliappan-nvidia]

Problem

When speculative decoding uses the native draft_model method (method="draft_model"), the draft
model runs eager every step — no CUDA-graph replay — even though its PIECEWISE graphs are captured
during dummy_run. The engine captures the draft graph but never dispatches to it, so each draft forward
is launch-bound (hundreds of thousands of cudaLaunchKernel calls) and can cost more than a full dense
verify, making draft_model self-speculative decoding net-negative.

Root cause

GPUModelRunner._check_and_update_cudagraph_mode() initializes the drafter's cudagraph-dispatcher
keys only for the EAGLE / extract-hidden-states paths:

# Initialize drafter's cudagraph dispatcher if using spec decode.
if self.speculative_config and (
    self.speculative_config.use_eagle()
    or self.speculative_config.uses_extract_hidden_states()
):
    assert isinstance(self.drafter, EagleProposer | DFlashProposer
                      | ExtractHiddenStatesProposer | Gemma4Proposer)
    self.drafter.initialize_cudagraph_keys(cudagraph_mode)

uses_draft_model() is excluded, so for method="draft_model" the drafter's keys_initialized stays
False, and CUDAGraphDispatcher.dispatch() returns CUDAGraphMode.NONE (eager) for every draft step.
This is inconsistent with every sibling gate in the same file, which already include uses_draft_model()
(drafter construction, attention-backend init, padded-drafter-batch, post-forward hidden-states gate).

Fix

Add uses_draft_model() to the drafter keys-init gate and DraftModelProposer to the assert:

if self.speculative_config and (
    self.speculative_config.use_eagle()
    or self.speculative_config.uses_draft_model()          # <-- ADD
    or self.speculative_config.uses_extract_hidden_states()
):
    assert isinstance(self.drafter, EagleProposer | DFlashProposer | DraftModelProposer
                      | ExtractHiddenStatesProposer | Gemma4Proposer)   # <-- add DraftModelProposer
    self.drafter.initialize_cudagraph_keys(cudagraph_mode)

The draft's PIECEWISE graphs are already captured (the capture block already includes
uses_draft_model()); this only enables dispatch to them. No new capture, no accuracy change.

Impact

Because the drafter's cudagraph keys are never initialized, the draft model runs eager every step, so
each draft forward is launch-bound (an nsys trace of the pre-fix path shows a per-step cudaLaunchKernel
storm). After the fix, the draft forward is dispatched to its already-captured PIECEWISE graph, removing
that per-step launch overhead. Acceptance length is unchanged (identical tokens), so the entire effect is
per-draft-step latency: without graph replay the draft step can cost more than the target verify, which
makes draft_model self-speculation net-negative; with replay it becomes a net speedup.

Test plan

• Fix ported function-level off current main.
• pre-commit (ruff / ruff-format / mypy / typos / SPDX) clean on the changed files.
• Sanity e2e (reviewer/CI): draft_model spec decode produces identical tokens vs eager-draft
  (accuracy unchanged) and the draft step is graph-replayed (no per-step launch storm).

AI assistance disclosure

This change was drafted with AI assistance (Cursor / Claude), per the contributing guide's "AI Assisted
Contributions" policy. All changed lines were reviewed and validated end-to-end by the author. Attribution
is included via a Co-authored-by: Claude commit trailer.

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[claude]

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[avalliappan-nvidia]

cc @benchislett @luccafong @MatthewBonanni @njhill — small draft_model spec-decode bugfix (init drafter CUDA-graph keys). CODEOWNERS auto-requests @njhill (worker); cc'ing spec-decode owners since the fix is in the draft_model dispatch path.