Support destructors for custom stateful ops

[samskalicky]

Description

Support destructors for custom stateful ops

• Adds virtual destructor for CustomStatefulOp base class
• Added new library C API _opCallDestroyOpState to call delete on CustomStatefulOp in custom library
• Calls delete on customStatefulOp instance in CustomStatefulOpWrapper destructor
• Adds static create API to CustomStatefulOp to create instances of CustomStatefulOp derived classes using new. This API takes variadic args and passes them to the constructor of the templated class. For example: CustomStatefulOp::create<MyStatefulGemm>(count, attrs);
• Adds an example destructor to the MyStatefulGemm class and calls the new create API
• Added check to ensure instances of CustomStatefulOp were created with the create API to ensure they are allocated with new (since delete will be called on destruction) or emit a suppressible warning message.
• Added an example suppressing the message in the MyStatefulTransposeRowSP class
• The MyStatefulTransposeCSR class is an example where the message is emitted.

Now, when MXNet calls the destructor for the CustomStatefulOpWrapper class it will subsequently delete the instance of the CustomStatefulOp allocated with new and call the inherited class's destructor

[mxnet-bot]

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

FYI @rondogency

[rondogency]

LGTM!

[mseth10]

LGTM