ZeRO Optimizer State Sharding

Adam stores two moment estimates per parameter, both in float32. A 7B-parameter model carries 56 GB of optimiser state. ZeRO stage 1 shards that across N ranks; each rank owns 1/N of the optimiser. After the local step the updated parameter shards broadcast back, every rank reconstructs the full model, and the next step begins. The win is a linear memory drop on the largest single allocation in the training stack.

Prerequisites: Phase 19 Track C lessons 42-49