fix SoftmaxOutput resource bug

[wkcn]

Description

Fix #14301

It seems that BackwardResource in src/operator/softmax_output-inl.h is not used in SoftmaxOutput since it is not a legacy operator.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Tiny change. The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Remove BackwardResource in src/operator/softmax_output-inl.h
• Add ResourceRequest in src/operator/softmax_output.cc
• Add the testcase for SoftmaxOutput.

Comments

SoftmaxOutput is not a legacy operator, which is registered by NNVM_REGISTER_OP rather than MXNET_REGISTER_OP_PROPERTY, so BackwardResource is not used for it.
Meanwhile, I check all operators. Only SoftmaxOutput has the problem.

[pengzhao-intel]

Thanks for the quick fix :)
@wkcn would you mind adding a test case?

[wkcn]

@pengzhao-intel I will add the test case for SoftmaxOutput. Thanks!

Edit: I have written the test case : )