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

Fix Merge Bugs

* 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

Fix some link errors about NNPACK.

refine tensor's conditional compilation, remove Tensor::numel_ and add DDim::size()

Unchecked errors could be handled by: cd go; gometalinter --vendor --disable-all --enable errcheck $(glide nv)

Enable tensor test

* Also fix unit test

Python Generate OpCreation Methods by OpProto

* add inputs

* add ut for multiple inputs

* fix AddToLayer

* op_desc -> op_proto

* CreateArgumentOffsetMap -> CreateInOutOffsetMap

* move CreateInOutOffsetMap from OperatorBase to op registry

* arg_idxs_ -> in_out_idxs_

* Implement InferShape and register them, give a stub Kernel method
  by LOG(INFO)

CMake `op_library` function

Update the dockerfile to build cross-compiling image for Android.

Refactor DDim's product() and add slice_ddim()

* add NDEBUG switch to PADDLE_ENFORCE

Fix fetch record from master