[Model Runner V2] Enable forcing a specific acceptance rate during rejection sampling (#38045)

Signed-off-by: Giancarlo Delfin <gdelfin@inferact.ai>

tests/v1/spec_decode/test_synthetic_rejection_sampler_utils.py

@@ -0,0 +1,34 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import pytest
+
+from vllm.v1.worker.gpu.spec_decode.synthetic_rejection_sampler_utils import (
+    compute_synthetic_rejection_sampler_params,
+)
+
+NUM_SPECULATIVE_STEPS = [1, 2, 3, 4, 5, 7, 10]
+ACCEPTANCE_RATES = [i / 100 for i in range(0, 100)]
+
+
+@pytest.mark.parametrize("num_speculative_steps", NUM_SPECULATIVE_STEPS)
+def test_compute_synthetic_rejection_sampler_params(num_speculative_steps: int):
+    """Test that the base acceptance rate and decay factor generated for
+    synthetic rejection sampling have a mean joint acceptance probability
+    that matches the desired acceptance rate."""
+    tol = 1e-9
+    for desired_acceptance_rate in ACCEPTANCE_RATES:
+        base_rate, decay_factor = compute_synthetic_rejection_sampler_params(
+            desired_acceptance_rate, num_speculative_steps, tol=tol
+        )
+
+        # Compute the mean of joint acceptance probabilities across
+        # all speculative positions.
+        joint_prob = 1.0
+        mean_joint = 0.0
+        for i in range(num_speculative_steps):
+            joint_prob *= base_rate * decay_factor**i
+            mean_joint += joint_prob
+        mean_joint /= num_speculative_steps
+
+        assert abs(desired_acceptance_rate - mean_joint) < 10 * tol
+        assert base_rate <= 1.0