add var name in optimizer

test=release/1.3

test=release/1.3

* Impl fp16 compute kernel for slice_op

* Use data() to replace mutable_data()

test=release/1.3

* expose peak gpu memory API to python test=develop

* add unittest for peak gpu memory monitoring test=develop

* add pybind change test=develop

* add mutex to gpu mem usage monitor test=develop

* update benchmark flag definition file test=develop

* tweak unittest for memory monitoring test=develop

test=develop

* Enable pool2d operator for a ngraph engine
test=develop

* Update
test=develop

* Refine beam_search_op to output an extra parent_idx tensor.
test=develop

* Fix the unittest test_beam_search_op.
test=develop

* Fix the merging mistake.
test=develop

test=develop