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depends #5122
for 2 consecutive linear layers, if first is sharded axis=0, then no all2all comms are needed between the 2 layers
if embedding is sharded -1, then activations are sharded going into transformer blocks and layer norms, resulting in a lot of extra all2all rounds
note that embedding is (in_features, out_features), so sharding 0 means that the entire embedding is fetched on the 1 device that has it (with the rest having 0 vectors) and all reduced. ideally, we would shard -1 and force it to all-gather afterwards, but there is no way to do this in the current api.
benchmarks: (note that only changing both embeddings and w1/w3 helps):
With just the embeddings change:
With just the w1/w3 change:
baseline: