fix custom op error when using auxiliary states

[piiswrong]

Description

(Brief description on what this PR is about)

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

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Intersting edge cases to note here

[yajiedesign]

https://github.com/piiswrong/mxnet/blob/52c509aaad86d282e08784b763f73c563b07ae7d/src/operator/custom/custom.cc#L243

change in_shape.size() to params.num_args