Make PyBind support C++ exception

Unify clang-format and add some missing clang-format

enable LoDTensor get number of elements at a specific level

* some enforce change

* add LoDTensor::NumElements(id,id)

* update

* init

* finish

* refacter interfaces

fix compiler warning from MKLDNNLayer and so on

refine mkldnn benchmark script

fix atomic issue on ONLYCPU mode

Fix doc of clip op.

Reduce elementwise warning

refine the catalog of FAQ

fix the conflicts of SoftmaxWithCrossEntropyOp with the latest dev branch.

Feature/pybind for protobuf desc

Fix error in unit test of ModifiedHuberLossOp

Use `bool` for PADDLE_ENFORCE, not int

Softmax with cross entropy op.