Skeleton Of fully connected operator

We put forward Op's inputs, outputs and output gradients into Grad
Op's inputs, and put forward Op's input gradients into Grad Op's output.
So Grad Op's `in_out_idx`, `input_format` and 'output format' need to be
rebuilt during Op creating.

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.

Change `in_out_idxs_` to shared_ptr

Add gometalinter errcheck to pre-commit-hook, fix all Go unchecked errors.

* `in_out_idxs_` shares between all operator instance in same type
  of operator.

Fix Merge Bugs

Net::DebugString() print what it is and invokes inner operators
DebugString() with a prefix '\t'.

* There is a merge conflict when merge PR #2914
* Develop and PR #2914 both add `DDim::size` method, but did not
  triger git merge conflict while merge.

Add skeletons of `mul`, `rowwise_add`, `sigmoid`, `softmax` ops

* Follow review comments to seperate Tensor Numpy interactive methods in
  tensor.h.
* Simplify logic for `CastToPyBufferImpl`, make it as one struct and in
  details namespace.
* Remove `Scope` expose in Python, since it currently is useless.
* Remove some debug functions.