[Core] Subclass ModelRunner to support cross-attention & encoder sequences (towards eventual encoder/decoder model support) (#4942)

Co-authored-by: Andrew Feldman <afeld2012@gmail.com>
Co-authored-by: Nick Hill <nickhill@us.ibm.com>

tests/distributed/test_basic_distributed_correctness_enc_dec.py

@@ -0,0 +1,101 @@
+"""For encoder/decoder models only:
+Compare the outputs of HF and distributed vLLM when using greedy sampling.
+
+Run:
+```sh
+cd $VLLM_PATH/tests
+
+pytest distributed/test_basic_distributed_correctness_enc_dec.py
+```
+"""
+
+import pytest
+
+from tests.models.utils import DecoderPromptType
+from vllm.utils import cuda_device_count_stateless
+
+from ..models.utils import check_logprobs_close
+from ..utils import fork_new_process_for_each_test
+
+
+@pytest.mark.skipif(cuda_device_count_stateless() < 2,
+                    reason="Need at least 2 GPUs to run the test.")
+@pytest.mark.parametrize("model, distributed_executor_backend", [
+    ("facebook/bart-large-cnn", "ray"),
+    ("facebook/bart-large-cnn", "mp"),
+])
+@fork_new_process_for_each_test
+def test_models(
+    model: str,
+    distributed_executor_backend: str,
+    hf_runner,
+    vllm_runner,
+    example_encoder_decoder_prompts,
+) -> None:
+    '''
+    Test vLLM BART inference on more than one GPU, comparing
+    outputs against HF as a baseline.
+
+    Fork a new process for each test, to prevent CUDA from
+    being re-initialized by successive tests within the same
+    process.
+
+    Arguments:
+
+    * model: the HF ID of the specific BART variant under test
+    * distributed_executor_backend
+    * hf_runner: HuggingFace (HF) test model runner
+    * vllm_runner: vLLM test model runner
+    * example_encoder_decoder_prompts: test fixture which provides a 
+                                        dictionary of dummy prompts
+    '''
+
+    dtype = "float"
+    max_tokens = 64
+    num_logprobs = 5
+
+    # Example inputs with non-trivial (i.e. not None/empty) encoder &
+    # decoder prompts.
+    test_prompts = example_encoder_decoder_prompts[DecoderPromptType.CUSTOM]
+
+    # NOTE: take care of the order. run vLLM first, and then run HF.
+    # vLLM needs a fresh new process without cuda initialization.
+    # if we run HF first, the cuda initialization will be done and it
+    # will hurt multiprocessing backend with fork method (the default method).
+    with vllm_runner(
+            model,
+            dtype=dtype,
+            tensor_parallel_size=2,
+            distributed_executor_backend=distributed_executor_backend,
+            enforce_eager=True,
+    ) as vllm_model:
+        vllm_outputs = vllm_model.generate_encoder_decoder_greedy_logprobs(
+            test_prompts, max_tokens, num_logprobs)
+
+    # Configuration settings for HF baseline
+    hf_kwargs = {
+        "top_k": None,
+        "num_beams": 1,
+        "repetition_penalty": 1.0,
+        "top_p": 1.0,
+        "length_penalty": 1.0,
+        "early_stopping": False,
+        "no_repeat_ngram_size": None,
+        "min_length": 0
+    }
+
+    with hf_runner(model, dtype=dtype,
+                   is_encoder_decoder_model=True) as hf_model:
+        hf_outputs = (hf_model.generate_encoder_decoder_greedy_logprobs_limit(
+            test_prompts,
+            max_tokens,
+            num_logprobs,
+            **hf_kwargs,
+        ))
+
+    check_logprobs_close(
+        outputs_0_lst=hf_outputs,
+        outputs_1_lst=vllm_outputs,
+        name_0="hf",
+        name_1="vllm",
+    )