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* Add serialization of reference dataset

* lint and missing file

* Fixes from reviewers

* responded to comments

* revert sdk change

This reverts commit 656d2676.

* [ci] fixed cpplint warnings about braces

* removed excess line

* some refines

* more omp refactoring

* format define

* fix merge bug

* some fixes

* fix some warnings

* Apply suggestions from code review

* Apply suggestions from code review

* remove dup codes

* fix compile warning for HDFS version

* cast to size_t

* Fix build error when HDFS is enabled.

* Replace string with char array to eliminate cpplint.

* fix many cpp lint errors

* indent

* fix bug

* fix more

* fix gpu

* more fixes

* added all necessary includes - fixed build/include_what_you_use error

* fixed the order of includes (build/include_order)

* fix include

* reduce dependency on header file

* fix build

* decode error description

* added break line char in log massages

* minor change

* unix line break

* Read and write datsets from hdfs.
* Only enabled when cmake is run with -DUSE_HDFS:BOOL=TRUE
* Introduces VirtualFile(Reader|Writer) to asbtract VFS differences