* a simplest SGD op

Refactorize Tensor to Eigen convesion

add crop layer

Feature/add op test

Skeleton Of fully connected operator

2. Add crop layer unitest
3. Fix bugs

Tensor should use PADDLE_ONLY_CPU macro

Add voc2012 dataset for image segment

Enable error clipping for fc layer

fix travis check style for Go

* Fc operator is a grouped operator, which combined by may internal
  operators.
* InferShape & Run a FC operator in Python.

Override Net::DebugString to invoke inner ops_

Add method converting Tensor to Eigen TensorMap

Make CreateOp in Plain C++ params

Read/Write a Tensor Python

* Not use __CUDACC__ macro, because malloc in GPU memory do not have to
  used in .cu file.
* Also, `PADDLE_ENFORCE` logic is wrong.