Mooncake Store Rollout Transfer

Transfer rollout tensors (tokens, loss_masks) via Mooncake Store instead of Ray object refs when rollout and training run on separate GPUs.

pip install mooncake-transfer-engine
mooncake_master --enable_http_metadata_server=true \
  --http_metadata_server_host=127.0.0.1 --http_metadata_server_port=18080
python train_async.py --transfer-backend mooncake_store

Optional: --mooncake-store-init-kwargs '{"master_server_addr":"127.0.0.1:50051"}' or MOONCAKE_* env vars.

Ported from slime PR #1709.