Fix the bug that F.SoftmaxCrossEntropy does not work properly when ignore_label is other than -1

[delta2323]

Fix #2715

[okuta]

How about following code?

ret = cuda.reduce(
  'S t, raw T log_y, int32 n_channel, raw T coeff, S ignore_label', 'T out',
  't == ignore_label ? T(0) : log_y[_j * n_channel + t]',
  'a + b', 'out = a * -coeff[0]', '0', 'crossent_fwd'
)(t, log_y.reduced_view(), log_y.shape[-1], self._coeff, self.ignore_label)

[delta2323]

Thank you for suggestion. I would like to know the reason you prefer this implementation. I noticed that some kernels in this function are implemented in the way you suggested. Is the reason that we should align the implementation to existing ones?

[okuta]

Your code generates cubin for each self.ignore_label.

[delta2323]

Got it. Thank you. I will change as you suggest.

[delta2323]

I fixed.

[okuta]

LGTM!