1. Change PODDeleter's template parameter 'PlaceType' to 'Place'.

2. Limit PODDeleter and Tensor::mutable_data()'s `T` to POD type.

* add cross-entropy-op

* add infershape and compute

* implement Infershape and compute of onehotcrossentropy op

fix client discover pserver context cancelled

It's already fixed by Wuyi's PR, but his PR may take some time to
merge, but I want to get this change in ASAP.

Update Memory ReadMe Doc

Copyright

fix recurrent_group parsing bug.

fix flowers dataset bug

fix bug in EigenVector::Flatten method

Fix croplayer compile err

ENH: unify PADDLE_ENFORCE for cublas, cudnn, curand

update tensor usage in sgd-op

cmake: fix problem that go_library is never rebuilt.

* a simplest SGD op

Refactorize Tensor to Eigen convesion

add crop layer

Feature/add op test

ATTENTION: some interfaces changed:
1. void Tensor::set_dims(const DDim& dims) ==> void Tensor::Resize(const DDim& dims).
2. void Tensor::ShareDataFrom(const Tensor& src)  ==> void Tensor::ShareDataWith(const Tensor& src)
3. DDim Tensor::dims() const ==> const DDim& Tensor::dims() const