not work for sparse embedding model

[muye5]

thanks for your great job. I found it didn't work for large scale model with huge sparse embedding parameters. for the apply_sparse function may be not very efficient as AdamOptimizer in tensorflow. So do you have a plan to re-implement the sparse version of adabound?

I compared the difference between this version and AdamOptimizer in TF. I think the key difference of efficiency is: AdamOptimizer of TF call the function training_ops.sparse_apply_adam() , this function accelerate the updating of sparse parameters.