Stabilize CTCLoss by initializing log_beta values to -inf, fixing comparisons, zeroing padding gradients #21392
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Summary:
as discussed at #21244, we
found some values in log_beta are not properly initialized. This diff will 1)
initialize all log_beta to -inf; 2) fix a tricky compare condition; 3) zero all
the gradient elements corresponding to padding to zero.
Offline experiments show that this diff can fix previous seen NaN loss.
Differential Revision: D15637977