A todo list for usability improvement for sparse tensor

[eric-haibin-lin]

throw exception on storage fallback

Currently when storage fallback happens, the log message is printed in the console, but it's very hard for the user to figure out when the fallback happens in the code even if they use NaiveEngine for debugging. We should throw an exception when storage fallback happens to help the user debug it.
https://github.com/apache/incubator-mxnet/blob/master/src/operator/operator_common.h#L526

missing storage fallback message/exception during ndarray copy

cast_storage may happen when the source and destination ndarray are of different storage types. Currently it's converted in silence and the user is not aware.

sparse vector with shape=(10)

#8817
Still under discussion. We should either fix it or at least not let VM crash.

distributed training tutorial for sparse

Maybe do this after kvstore=device is done.

Testing

#8709
#8542
#8980

[ZiyueHuang]

+1 for distributed training tutorial. I can take this task after #8732 is done.