fierceX opened a new pull request #14673: [BUGFIX] fix ELU function will appear nan when calculating the gradient URL: https://github.com/apache/incubator-mxnet/pull/14673 ## Description ## fix ELU function will appear `nan` when calculating the gradient e.g: ```python elu = nn.ELU() elu.initialize() x = nd.ones((2, 3)) x.attach_grad() x[0] = 100 x[1] = -1 with autograd.record(): y = elu(x) y.backward() print(y) print(x.grad) ``` output is ``` [[100. 100. 100. ] [ -0.63212055 -0.63212055 -0.63212055]] <NDArray 2x3 @cpu(0)> [[ nan nan nan] [0.36787945 0.36787945 0.36787945]] <NDArray 2x3 @cpu(0)> ``` After experiments, it was found that the `where` and the `exp` were used together and that this bug would occur e.g: ```python x = nd.ones((2, 3)) x.attach_grad() x[0] = 100 x[1] = 5 with autograd.record(): y = nd.where(x > 0, x, nd.exp(x)) y.backward() print(y) print(x.grad) ``` output: ``` [[100. 100. 100.] [ 5. 5. 5.]] <NDArray 2x3 @cpu(0)> [[nan nan nan] [ 1. 1. 1.]] <NDArray 2x3 @cpu(0)> ``` When the `exp` calculation appears `inf`, even if `where` does not select the value, but the gradient will still be `nan`,so this PR does not completely fix the problem, but based on this, modified the ELU calculation method so that it does not appear `nan`
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