Modal Clusters are generally available

Introduces Modal Clusters, a new primitive for multi-node workloads, accessible via the `@modal.clustered` decorator. This enables communication between nodes via InfiniBand verbs at up to 6.4 Tbps, automatically configured for PyTorch and NCCL. The post details the implementation of a new gang scheduler with an observe-plan-act loop for atomic N-node placement. It also describes the integration of RDMA support into gVisor, including upstreaming changes to the OSS project, to enable secure multi-tenant RDMA communication. The post highlights customer use cases for training and inference at scale.