Changes to layout selection heuristic for cases like XLA frontend where fprop layouts are not propagated to bprop graph (#769)

* fix relu(conv); the old test was fusing conv(relu) instead of running both separetely

* fix segfault due to initiating a vector of result vectors w/ a number of arguments

* [Py] Update serialization

* Update docstring

* Add UT for serialize model

* fixed two bugs.

* refactor infer shape.

* clang format.

* reverted some changes.

* moved infer_convolution_output_shape() to outside class

* moved infer_convolution_output_shape to op::util namespace

*  split cpu_fusion into cpu_fusion and cpu_test; clean up headers

* fix formatting

* add new line to tensor_mask.hpp

* move CoreFusion to ngraph/pass

* fix incorrect header path

* add nd reshape

* compiler and no crash with wrong result version

* change output_stride to trans_stride, which transform input idx to output idx

* using vector instead of c array

* remove delete

* using const and reference to pass string and array

* change 'unimplement' comments, remove extra indents

* format and cast size_t to int

* tensor (sequence) mask

* convert sequence to match input type

* update API to take batch axis

* registers nnp tester

* check output size

* rename to nnp tester

* revert changes in graph_util

* update CentOS requs

* doc link imports title of section so remove redundant word

* CentOS getting its own section for install

* don't need both http and ssh clone for centos

* don't need to be root to install ngraph

* Integrate PR comments

* Update section on CentOS install for nGraph with PR feedback

* CMake: Generate code for the host CPU

* CMake: Separate machine-specific flag

* Increase tolerance for convolution tests

* test graph.

* rnn matrix fusion wip.

* WIP.

* matrix fused.

* clean up.

* additional refactor.

* fixed merge errors.

* testing.

* added test case.

* more testing.

* more testing.

* bias wip.

* debuggging.

* fusing bias too.

* disabled debug outputs.

* removed debugging.

* removed testing models.

* removed debug models.

* reset nbench.

* clean up and testing.

* removed debug code.

* updated comments and var names.

* rename var.

* removed debug code.

* removed debug code.

* fixed compiler warnings.

* refactored test.

* Added const to parameter.

* address some of the review comments.

* added comments and addressed reviews.

updating ngraph theme with IntelNeoSans font on headings, better OFL font for doc readability, better rendering of certain text, etc (#727)

* Normalize op implementations

* update custom backend ops

* Add test for ng.absolute

- remove unnecesary repetitive call to as_node

* Add Acos operation with UT.

- update docsrings
- refactor and parameterize unit tests

* Review refactoring fix.

* Review refactoring fix part 2.

* Add Asin and Atan nGraph operation wrappers.

* [Py] Add UT for avg_pooling_3d(Cover uncontigous data input)

Issue #725

* make sure we don't get insidious perf bugs due to doule op::Results
* fix graph_partition tests