Reserve the KV cache address range with CUDA virtual memory, commit a
minimal prefix before CUDA graph capture, measure real post-capture
memory usage, then commit the final KV cache size with stable tensor
addresses. Opt-in via --enable-extensible-kv-cache.
Squashed pick of vllm-project/vllm#47363.
Co-authored-by: Zhuohan Li <zhuohan123@gmail.com>
Signed-off-by: Nick Hill <nickhill@us.ibm.com>
