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Issue
In case there are multiple output values and one of them is loss, some user reported the following error:
Note this part:
Output gradient: ('None',)
I can repro the issue in local_test_c10d_bwd.py, if I change the output to:
Cause
The above issue is caused by the fixed setting in the else case:
This tuple
(None,)
should have variable length depending on the output.Fix
Only update
bwd_kwargs["output_grads"]
when we have actually received gradients; otherwise, use the tuple prepared during IR phase, i.e.bwd_node.kwargs["output_grads"]
, which may look like(None, None)
if there are two outputs.