Account for memory usage of other processes

[maxdebayser]

FIX #18854

In determine_available_memory, the initial memory usage should be taken into account.

Consider this script running on a 80GB A100:

from vllm import LLM, SamplingParams

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=0.8, top_p=0.95)

# makes sure gpu_memory_utilization is per-instance limit,
# not a global limit
llms = [
    LLM(model="facebook/opt-125m",
        gpu_memory_utilization=0.3,
        enforce_eager=True) for i in range(3)
]
for llm in llms:
    outputs = llm.generate(prompts, sampling_params)
    for output in outputs:
        prompt = output.prompt
        generated_text = output.outputs[0].text
        print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")

If I print the memory metrics without my changes, the second process already crashes because the estimated available memory is negative:

self.init_gpu_memory=84530692096
total_allocated_bytes=844103680
torch_allocated_bytes=278004224
total_gpu_memory=84974239744
peak_memory=770516480
non_torch_allocations=566099456
self.cache_config.gpu_memory_utilization=0.3
available_memory=24155655987

self.init_gpu_memory=59029782528
total_allocated_bytes=26345013248
torch_allocated_bytes=278004224
total_gpu_memory=84974239744
peak_memory=770516480
non_torch_allocations=26067009024
self.cache_config.gpu_memory_utilization=0.3
available_memory=-1345253580

Taking the initial memory into account, the 3 processes can run and the estimated memory is an exact match for the 3 processes:

self.init_gpu_memory=84530692096
total_allocated_bytes=844103680
torch_allocated_bytes=278004224
total_gpu_memory=84974239744
peak_memory=770516480
non_torch_allocations=122551808
self.cache_config.gpu_memory_utilization=0.3
available_memory=24599203635

self.init_gpu_memory=58576797696
total_allocated_bytes=26797998080
torch_allocated_bytes=278004224
total_gpu_memory=84974239744
peak_memory=770516480
non_torch_allocations=122551808
self.cache_config.gpu_memory_utilization=0.3
available_memory=24599203635

self.init_gpu_memory=32622903296
total_allocated_bytes=52751892480
torch_allocated_bytes=278004224
total_gpu_memory=84974239744
peak_memory=770516480
non_torch_allocations=122551808
self.cache_config.gpu_memory_utilization=0.3
available_memory=24599203635

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