Add Shape and Scalar type to ChxVMVar

[take-cheeze]

• closes Add scalar/shape types to XCVMVar #18
• Only adds enum and getter to ChxVMVar
• Adding abseil for variant type since currently chainer-compiler uses C++14

[take-cheeze]

Making the output of Shape operator chainerx::Shape seems to be a good start point to use this

[shinh]

Thanks! I'm guessing introducing these types would make the following faster:

$ PYTHONPATH=ch2o python3 ch2o/tests/model/EspNet_E2E.py --recipe csj_medium --gen csj_medium --gpu
$ ./build/tools/run_onnx --test csj_medium_backprop --backprop -d cuda -I 10 --fuse_operations --use_nvrtc
Average elapsed: 192.045 msec

Hmm it used to be 100ms or so. Maybe there were some regressions.

The following command runs the same network by Chainer for reference:

$ PYTHONPATH=ch2o python3 ch2o/tests/model/EspNet_E2E.py --recipe csj_medium --run --gpu
Elapsed: 4776.919364929199 msec
Elapsed: 203.45425605773926 msec
Elapsed: 209.4278335571289 msec
Elapsed: 212.3851776123047 msec
Elapsed: 209.69533920288086 msec
Elapsed: 223.52290153503418 msec
Elapsed: 209.55896377563477 msec
Elapsed: 211.23909950256348 msec
Elapsed: 208.7113857269287 msec
Elapsed: 209.62882041931152 msec
Average elapsed: 210.84708637661404 msec