[mlpack-git] [mlpack] improve speed of SparseAutoencoder and make it more flexible (#451)

[stereomatchingkiss]

Finally, all jobs have been done. I do some change on FFN(do not take the reference of outputLayer but add a public api to users, this make serialization become easier).

I use FeedForward to get the Evaluate value(Evaluate api do not give the value)
Call FeedForward and FeedBackWard to get the gradient value

Please check the test cases, they test with the same data(except of the gradient part) but with different api, I hope my implementation meet the requirement of the semantic. 

If you think everything going well, you can merge it now. Thanks for taking time review

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Reply to this email directly or view it on GitHub:
https://github.com/mlpack/mlpack/pull/451#issuecomment-174543008
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