* Add DataFeeder

A v2 API like data feeder for book demos.
We can feed data directly from reader.

* Fix CI

* Add an unittest for while/rnn op forward

* Add unittest for raw while op backward

* Fix CI

* Add hinge loss op

* Update hinge-loss equation for proper latex

* fix grpc compile warn

* update

* -Wnon-virtual-dtor -> -Wno-non-virtual-dtor

* change GPU memory allocating policy

* fix potential overflow bug

Add version and commit information in capi config.h and use unofficial glog for Android API < 21. (#6113)

* Automatically configure the version and commit information in capi.

* Use the unofficial glog repository for building for Android (API < 21).

Also, change the `labels` to `label` for api consistency

* Enable the case N != ldc in EigenBlasGemm.

* Use MemoryHandle instead of direct calling of posix_memalign to alloc temporary memory.

* Use Eigen's slice() instead of a temporary memory.

* Add if-else for different cases in EigenBlasGemm (for N ?= ldc).

* Fix Proformance problem of enforce

* Fix missing `;` in code

* Fix CI

* add gflags for C++ unittest

Fix #6087

* add back print_operators_doc

* fix style check

* fix style check