Fix potential divide by zero for CostInferenceForRowWiseSparseAdagrad
Summary: Fix the potential divide by zero error in CostInferenceForRowWiseSparseAdagrad, when n has zero elements
Test Plan:
Ran buck test caffe2/caffe2/python/operator_test:adagrad_test
Result: https://our.intern.facebook.com/intern/testinfra/testrun/562950122086369
Reviewed By: idning
Differential Revision: D23520763
fbshipit-source-id: 191345bd24f5179a9dbdb41c6784eab102cfe89c
