Bug-fixes

Fix I/O multiprocessing for too many open file handles (#8904), race condition (#8995), deadlock (#9126).
Fix image IO integration with OpenCV 3.3 (#8757).
Fix Gluon block printing (#8956).
Fix float16 argmax when there is negative input. (#9149)
Fix random number generator to ensure sufficient randomness. (#9119, #9256, #9300)
Fix custom op multi-GPU scaling (#9283)
Fix gradient of gather_nd when duplicate entries exist in index. (#9200
Fixed incorrect gradient of gather_nd (https://github.com/apache/incubator-mxnet/pull/9200)
Fixed image io for opencv3.3 (https://github.com/apache/incubator-mxnet/pull/8757
)

New Features

Added support for standard SGD optimizer with momentum with `row_sparse` gradient.
Added `sparse.dot(dense, csr)` operator

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