Support kAddTo for softmax backward

[haojin2]

Description

Fix for issue #11411.

Checklist

Essentials

• 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

• kAddTo in backward pass

Comments

Note: No need to add kAddTo support for forward pass, actually there's no way to specify req to be kAddTo for forward computation.
New version of test_new_softmax passed more than 10000 trials on both CPU & GPU:

MXNET_TEST_COUNT=10000 nosetests --verbose tests/python/unittest/test_operator.py:test_new_softmax
/home/ubuntu/anaconda3/lib/python3.6/site-packages/urllib3/contrib/pyopenssl.py:46: DeprecationWarning: OpenSSL.rand is deprecated - you should use os.urandom instead
  import OpenSSL.SSL
/home/ubuntu/anaconda3/lib/python3.6/site-packages/nose/util.py:453: DeprecationWarning: inspect.getargspec() is deprecated, use inspect.signature() or inspect.getfullargspec()
  inspect.getargspec(func)
[INFO] Setting module np/mx/python random seeds, use MXNET_MODULE_SEED=1585858917 to reproduce.
test_operator.test_new_softmax ... ok

----------------------------------------------------------------------
Ran 1 test in 737.356s

OK

MXNET_TEST_COUNT=10000 nosetests --verbose tests/python/gpu/test_operator_gpu.py:test_new_softmax
/home/ubuntu/anaconda3/lib/python3.6/site-packages/urllib3/contrib/pyopenssl.py:46: DeprecationWarning: OpenSSL.rand is deprecated - you should use os.urandom instead
  import OpenSSL.SSL
/home/ubuntu/anaconda3/lib/python3.6/site-packages/nose/util.py:453: DeprecationWarning: inspect.getargspec() is deprecated, use inspect.signature() or inspect.getfullargspec()
  inspect.getargspec(func)
[INFO] Setting module np/mx/python random seeds, use MXNET_MODULE_SEED=1970091674 to reproduce.
test_operator_gpu.test_new_softmax ... ok

----------------------------------------------------------------------
Ran 1 test in 904.984s

OK

[haojin2]

Experiencing some bugs on GPU side, changing to WIP.

[eric-haibin-lin]

any update?

[haojin2]

@eric-haibin-lin There's a weird bug that I'm still investigating...

[eric-haibin-lin]

Maybe also add 'null' to the list?

[haojin2]

Done

[apeforest]

@haojin2 I am curious what the bug you experienced earlier was and how did you fix it? I only see one commit in this PR. Thanks!

[haojin2]

@apeforest it was only some typo

[apeforest]

Thanks for clarifying. LGTM.

[apeforest]

@anirudh2290 @sandeep-krishnamurthy Can you help merge this PR at your convenience? Thanks

[anirudh2290]

do we want to reenable cudnn_forward for ctcloss op ?

[haojin2]

It's not within the scope of this PR, this PR is purely for fixing the problem with the request type the backward of softmax can handle. I can probably take a look into the problem later though, but I think this PR should be good to merge.