Fix test_kv_sharing_fast_prefill flakiness (#22038)

Signed-off-by: Yong Hoon Shin <yhshin@meta.com>

tests/v1/e2e/test_kv_sharing_fast_prefill.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-import gc
 import random
 from typing import Optional, Union
 
@@ -10,6 +9,7 @@ import torch
 
 from vllm import LLM, SamplingParams
 from vllm.config import CompilationConfig, CompilationLevel
+from vllm.distributed import cleanup_dist_env_and_memory
 from vllm.forward_context import get_forward_context
 from vllm.model_executor.models.gemma3n import Gemma3nForConditionalGeneration
 from vllm.model_executor.models.registry import ModelRegistry
@@ -18,6 +18,9 @@ from vllm.sequence import IntermediateTensors
 
 from ...utils import fork_new_process_for_each_test
 
+# global seed
+SEED = 42
+
 
 class TestGemma3nForConditionalGeneration(Gemma3nForConditionalGeneration):
 
@@ -95,8 +98,25 @@ def test_prompts():
     return prompts
 
 
+def cleanup(llm: LLM, compilation_config: CompilationConfig):
+    # hacky: below lines are required to free up memory for the next test
+    # when setting VLLM_ENABLE_V1_MULTIPROCESSING=0, del llm is not sufficient
+    # TODO(sarckk): when enforce_eager=False, memory is not freed:
+    # find out why and re-enable test for enforce_eager=False case
+    llm_engine = llm.llm_engine.engine_core.engine_core
+    model_runner = llm_engine.model_executor.driver_worker.worker.model_runner
+    del model_runner.model
+    del model_runner.kv_caches
+    del compilation_config.static_forward_context
+    compilation_config.static_forward_context = {}
+
+    del llm
+    torch.cuda.empty_cache()
+    cleanup_dist_env_and_memory()
+
+
 @fork_new_process_for_each_test
-@pytest.mark.parametrize("enforce_eager", [True, False])
+@pytest.mark.parametrize("enforce_eager", [True])
 def test_kv_sharing_fast_prefill(
     monkeypatch: pytest.MonkeyPatch,
     enforce_eager: bool,
@@ -115,23 +135,28 @@ def test_kv_sharing_fast_prefill(
     with monkeypatch.context() as m:
         m.setenv("VLLM_USE_V1", "1")
 
+        # Make scheduling deterministic for reproducibility
+        m.setenv("VLLM_ENABLE_V1_MULTIPROCESSING", "0")
+
         llm = LLM(
             model="google/gemma-3n-E2B-it",
             enforce_eager=enforce_eager,
             compilation_config=compilation_config,
+            seed=SEED,
         )
         ref_responses = llm.generate(test_prompts, sampling_params)
 
-        del llm
-        gc.collect()
-        torch.cuda.empty_cache()
+        cleanup(llm, compilation_config)
 
         llm = LLM(model="google/gemma-3n-E2B-it",
                   enforce_eager=enforce_eager,
                   compilation_config=compilation_config,
+                  seed=SEED,
                   kv_sharing_fast_prefill=True)
         optimized_responses = llm.generate(test_prompts, sampling_params)
 
+        cleanup(llm, compilation_config)
+
         misses = 0
 
         for ref_response, optimized_response in zip(ref_responses,