Add optimizer priming for dist chkpt #6572
Merged
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See also: #6546
The optimizer state must be primed before it can be restored. Optimizer state isn't materialized until the first
optim.step
call, so to restore optimizer state before resuming training, a dummy step is needed.This PR introduces the
prime_optimizer
API, which will run a dummy optimizer step with zeroed gradients. The gradient sharding is copied from the parameters to ensure the resulting sharding is the same.