Adam solver

[ronghanghu]

Carried on Adam solver (originally #2856) for merge.

I completed the tests and rebased it to latest master.

Authorship belongs to @PatWie, and is preserved in git commit.

Original message in #2856 :

This commit implements the Adam solver by Kingma et. al for CPU and GPU. All solver parameters are defined in the caffe.proto. This also adds an example for the MNIST dataset.

As you may see, now both solver.cpp and test_gradient_based_solver.cpp are growing to 1000+ lines. This problem will be addressed in #2890.

[ronghanghu]

Encountered error larger than margin on solver tests. Perhaps this is simply numerical issue. Looking into details.
It was a mistake I made when implementing test code ComputeLeastSquaresUpdate for Adam, where I forgot to power beta1 and beta2 by t. Now everything is fine.

[ronghanghu]

@shelhamer @jeffdonahue @philkr @PatWie Please take a look if you have time. I think this should be ready to merge.

This is last piece of the solver trilogy in #2860. After merging this one, we can address #2890.

[jeffdonahue]

We need to cite the ADAM paper somewhere*. I suggest putting a reference here, e.g. in a doxygen formatted comment like this. Eventually it would also be good to add sections to the solver tutorial on these new solvers, where the reference should also then be added.

*We probably also need to go back and add references for some of the other recently merged solvers.

[jeffdonahue]

Thanks for the rebase @ronghanghu and thanks @PatWie for the original implementation! See above comment; otherwise looks good.

[ronghanghu]

Citation added for Adam.

Eventually it would also be good to add sections to the solver tutorial on these new solvers, where the reference should also then be added.
*We probably also need to go back and add references for some of the other recently merged solvers.

Let's address that in #2890 .

[jeffdonahue]

No need to create shared_ptrs for these val_* variables, is there? (I suggest using the raw pointer, e.g. Blob<Dtype>* val_m = this->history_[param_id].get();.)

[ronghanghu]

Yes, you are right. I should use raw pointers.

[jeffdonahue]

Thanks for adding the citation. After a final glance I noticed the one other thing I commented above; sorry about not noticing before. Feel free to merge after addressing that.

[PatWie]

Looks good.

[ronghanghu]

Changed from shared ptrs to raw ptrs in AdamSolver<Dtype>::ComputeUpdateValue.