Make ci pass

Make test_CompareSparse eat less memory and faster.

Add explicit to some constructors

Re-structure source code of type Attribute

SKip cpplint if source is not changed.

Simplify test_matrixCompare

Move constants from framework::OperatorBase to framework::

convert dataset into recordio format

Enforce add more helpers

enable defining sub-sequence data in test layer gradients.

Disable MKL and OpenMP by default

* because it uses `numpy.flip` in `test_image.py`

Initialize Gradient Checker