[v1.x Backport] Fix softmax, logsoftmax failed on empty ndarray (#18602)

[bgawrych]

Description

Backport of two changes:

• Fix softmax, logsoftmax failed on empty ndarray (Fix softmax, logsoftmax failed on empty ndarray #18602)
• fix npx.softmax for 0-sized inputs ([numpy] fix npx.softmax for 0-sized inputs #18158)

Third commit "Fix softmax, logsoftmax backward failed on empty ndarray (#18710)" is not backported
directly from master branch because fix was covered by other PR improving unittests (#18694).

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• 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)
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[mxnet-bot]

Hey @bgawrych , Thanks for submitting the PR
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[bgawrych]

@mxnet-bot run ci [edge, unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [edge, unix-gpu]

[bgawrych]

@mxnet-bot run ci [edge, unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [edge, unix-gpu]

[bgawrych]

@TaoLv @PatricZhao Can you take a look?
CC: @Yiyan66 as owner of the fix npx.softmax for 0-sized inputs (#18158)

[TaoLv]

@bgawrych I'm rebasing your branch to the latest of v1.x.

[bgawrych]

@mxnet-bot run ci [website]

[mxnet-bot]

Jenkins CI successfully triggered : [website]

[TaoLv]

Not sure what's the merge policy of v1.x. Github keeps saying this branch is out of date w ith the base branch so we cannot merge it.

[bgawrych]

@mxnet-bot run ci [unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu]

[bgawrych]

@mxnet-bot run ci [unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu]

[bgawrych]

@mxnet-bot run ci [all]

[mxnet-bot]

Jenkins CI successfully triggered : [edge, centos-gpu, sanity, unix-gpu, clang, centos-cpu, unix-cpu, miscellaneous, website, windows-gpu, windows-cpu]

[bgawrych]

@TaoLv @PatricZhao I think that the problem with out of date branch is solved

[TaoLv]

@bgawrych Thanks. Now it's merged.