enable ddp find_unused_parameterse #111
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The DDP takes the input and passes it to the local model, and then analyzes the output from the local model if find_unused_parameters is set to True. This mode allows running backward on a subgraph of the model, and DDP finds out which parameters are involved in the backward pass by traversing the autograd graph from the model output and marking all unused parameters as ready for reduction. During the backward pass, the Reducer would only wait for unready parameters, but it would still reduce all buckets. Marking a parameter gradient as ready does not help DDP skip buckets as for now, but it will prevent DDP from waiting for absent gradients forever during the backward pass. Note that traversing the autograd graph introduces extra overheads, so applications should only set find_unused_parameters to True when necessary. |
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