[core] LLM.collective_rpc interface and RLHF example (#12084)

Signed-off-by: youkaichao <youkaichao@gmail.com>

examples/offline_inference/rlhf.py

@@ -0,0 +1,191 @@
+"""
+a simple demonstration of RLHF with vLLM, inspired by
+the OpenRLHF framework https://github.com/OpenRLHF/OpenRLHF .
+It follows the design that, training processes and inference processes
+are different, and they live on different GPUs.
+Training processes send prompts to inference processes to generate data,
+and also synchronize the weights of the model by broadcasting the weights
+from the training process to the inference process.
+Note that this is a simple demonstration of one training instance and one
+inference instance. In practice, there could be multiple training instances
+and multiple inference instances. For the full implementation, please refer
+to the OpenRLHF framework.
+"""
+import os
+
+import ray
+import torch
+from ray.util.placement_group import placement_group
+from ray.util.scheduling_strategies import PlacementGroupSchedulingStrategy
+from transformers import AutoModelForCausalLM
+
+from vllm import LLM, SamplingParams, configure_as_vllm_process
+from vllm.utils import get_ip, get_open_port
+from vllm.worker.worker import Worker
+
+
+def stateless_init_process_group(master_address, master_port, rank, world_size,
+                                 device):
+    """
+    vLLM provides `StatelessProcessGroup` to create a process group
+    without considering the global process group in torch.distributed.
+    It is recommended to create `StatelessProcessGroup`, and then initialize
+    the data-plane communication (NCCL) between external (train processes) 
+    and vLLM workers.
+    """
+    from vllm.distributed.device_communicators.pynccl import PyNcclCommunicator
+    from vllm.distributed.utils import StatelessProcessGroup
+    pg = StatelessProcessGroup.create(host=master_address,
+                                      port=master_port,
+                                      rank=rank,
+                                      world_size=world_size)
+    pynccl = PyNcclCommunicator(pg, device=device)
+    return pynccl
+
+
+class MyWorker(Worker):
+    """
+    The `MyWorker` class inherits from `Worker` to provide custom functions.
+    For simplicity, we define the `MyWorker` class in this self-contained 
+    script. Normally, we should define the `MyWorker` class in a separate 
+    file and pass the qualified name of the class to the `worker_cls` 
+    parameter.
+    """
+
+    def init_weight_update_group(self, master_address, master_port,
+                                 rank_offset, world_size):
+        from vllm.distributed.parallel_state import get_world_group
+        rank = get_world_group().rank + rank_offset
+        self.model_update_group = stateless_init_process_group(
+            master_address,
+            master_port,
+            rank,
+            world_size,
+            self.device,
+        )
+
+    def update_weight(self, name, dtype, shape):
+        weight = torch.empty(shape, dtype=dtype, device="cuda")
+        self.model_update_group.broadcast(weight,
+                                          src=0,
+                                          stream=torch.cuda.current_stream())
+
+        self.model_runner.model.load_weights(weights=[(name, weight)])
+
+        del weight
+
+    def check_weights_changed(self):
+        """
+        Check if the weights are updated to 0.
+        """
+        weights_updated = True
+        for name, p in self.model_runner.model.named_parameters():
+            weights_updated = weights_updated and torch.allclose(
+                p, torch.zeros_like(p))
+        return weights_updated
+
+
+class MyLLM(LLM):
+
+    def __init__(self, *args, **kwargs):
+        # a hack to make the script work.
+        # stop ray from manipulating CUDA_VISIBLE_DEVICES
+        # at the top-level
+        del os.environ["CUDA_VISIBLE_DEVICES"]
+        super().__init__(*args, **kwargs)
+
+
+"""
+Start the training process, here we use huggingface transformers 
+as an example to hold a model on GPU 0.
+
+It is important for all the processes outside of vLLM to call
+`configure_as_vllm_process` to set some common environment variables
+the same as vLLM workers.
+"""
+configure_as_vllm_process()
+
+train_model = AutoModelForCausalLM.from_pretrained("facebook/opt-125m")
+train_model.to("cuda:0")
+"""
+Start the inference process, here we use vLLM to hold a model on GPU 1 and 
+GPU 2. For the details on how to use ray, please refer to the ray 
+documentation https://docs.ray.io/en/latest/ .
+"""
+os.environ["CUDA_VISIBLE_DEVICES"] = "1,2"
+ray.init()
+
+pg_inference = placement_group([{"GPU": 1, "CPU": 0}] * 2)
+ray.get(pg_inference.ready())
+scheduling_inference = PlacementGroupSchedulingStrategy(
+    placement_group=pg_inference,
+    placement_group_capture_child_tasks=True,
+    placement_group_bundle_index=0,
+)
+"""
+launch the vLLM inference engine.
+here we use `enforce_eager` to reduce the start time.
+"""
+llm = ray.remote(
+    num_cpus=0,
+    num_gpus=0,
+    scheduling_strategy=scheduling_inference,
+)(MyLLM).remote(
+    model="facebook/opt-125m",
+    enforce_eager=True,
+    worker_cls=MyWorker,
+    tensor_parallel_size=2,
+    distributed_executor_backend="ray",
+)
+
+# Generate texts from the prompts.
+prompts = [
+    "Hello, my name is",
+    "The president of the United States is",
+    "The capital of France is",
+    "The future of AI is",
+]
+
+sampling_params = SamplingParams(temperature=0)
+
+outputs = ray.get(llm.generate.remote(prompts, sampling_params))
+
+for output in outputs:
+    prompt = output.prompt
+    generated_text = output.outputs[0].text
+    print(f"Prompt: {prompt!r}, "
+          f"Generated text: {generated_text!r}")
+
+# set up the communication between the training process
+# and the inference engine.
+master_address = get_ip()
+master_port = get_open_port()
+
+handle = llm.collective_rpc.remote("init_weight_update_group",
+                                   args=(master_address, master_port, 1, 3))
+model_update_group = stateless_init_process_group(master_address, master_port,
+                                                  0, 3, torch.device("cuda:0"))
+ray.get(handle)
+
+# simulate training, modify the weights of the model.
+for name, p in train_model.named_parameters():
+    p.data.zero_()
+
+# sync weight from the training process to the inference engine.
+for name, p in train_model.named_parameters():
+    handle = llm.collective_rpc.remote("update_weight",
+                                       args=(name, p.dtype, p.shape))
+    model_update_group.broadcast(p, src=0, stream=torch.cuda.current_stream())
+    ray.get(handle)
+
+# check if the weights are updated.
+assert all(ray.get(llm.collective_rpc.remote("check_weights_changed")))
+
+# use the updated model to generate texts, they will be nonsense
+# because the weights are all zeros.
+outputs_updated = ray.get(llm.generate.remote(prompts, sampling_params))
+for output in outputs_updated:
+    prompt = output.prompt
+    generated_text = output.outputs[0].text
+    print(f"Prompt: {prompt!r}, "
+          f"Generated text: {generated_text!r}")