Fix softmax_cross_entropy

[kevinthesun]

Description

Fix softmax_cross_entropy operator "Not enough argument to call operator softmax_cross_entropy" issue #6874

Checklist

Essentials

• Passed code style checking (make lint)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• For user-facing API changes, API doc string has been updated.
• To my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Fix "Not enough argument to call operator softmax_cross_entropy" issue for softmax_cross_entropy operator.
• Change output to be shape (batch_size,), which is consistent with gluon softmax loss.

[piiswrong]

softmax_cross_entropy's output currently has shape (1,)
It should behave like gluon.loss.SoftmaxCrossEntropyLoss instead.
Since this is currently broken and no one is using it, we should take the chance to fix it

[kevinthesun]

Got it. I'll try to fix it.

[piiswrong]

closing due to inactive

[eric-haibin-lin]

This issue breaks existing examples #6874
We should fix this

[eric-haibin-lin]

@kevinthesun you can remove the sumall_except_dim at https://github.com/apache/incubator-mxnet/blob/master/src/operator/loss_binary_op-inl.h#L80 to make sure it doesn't return scalar result.
Also update the infer shape function

[Roshrini]

+1 Currently https://github.com/apache/incubator-mxnet/tree/master/example/model-parallel/lstm example breaks because of this issue.

[kevinthesun]

@eric-haibin-lin Will work on this today.

[kevinthesun]

@piiswrong @eric-haibin-lin @Roshrini

[ehsanmok]

Any update for this fix?

[eric-haibin-lin]

@kevinthesun any update?

[kevinthesun]

Already fixed.

[CircleXing001]

@kevinthesun why it reproduce

[tao-sun]

I use docker python:1.1.0_gpu_cuda8 and the same error is reproduced.