[Performance] Add prefetch for checkpoints to OS page cache (#36012)

Signed-off-by: Artem Perevedentsev <aperevedents@nvidia.com>

vllm/config/load.py

@@ -62,6 +62,9 @@ class LoadConfig:
       This is recommended for models on network filesystems (e.g., Lustre, NFS)
       as it avoids inefficient random reads, significantly speeding up model
       initialization. However, it uses more CPU RAM.
+    - "prefetch": Checkpoint files are read into the OS page cache before
+      workers load them, speeding up the model loading phase. Useful on
+      network or high-latency storage.
     - "torchao": Weights are loaded in upfront and then reconstructed
       into torchao tensor subclasses. This is used when the checkpoint
       was quantized using torchao and saved using safetensors.