onnxruntime

Add front-end MNIST test (#3231)

* add frontend minst test

* to use torch nightly with torchvision

* remove incorrect comment per reviewer's comment

* experiment torchvision import failure

* experiment install_deps.sh

* more experiment install_deps.sh

* experiment install_deps.sh with --upgrade

* Experiment with install_deps.sh.

* Experiment with install_ubuntu.sh.

* Use Ubuntu 18.04 and Python 3.6 for CI.

* Update cmake version for CI.

* Install MPI on Ubuntu 18.04 for CI.

* Increase tolerance for MNIST test.

* Go back to Ubuntu 16.04 for CI, fix installing from deadsnakes ppa.

* Clean-up.

* Update ort_trainer.py from ort_training.

* Get default Ubuntu Python ver back to 3.5.

* Add underscore to opset_version parameter name in ORTTrainer constructor.

* Move loss/model wrap before the call for sample output.

* Update expected values for MNIST test.

Co-authored-by: liqun <liqun@OrtTrainingDev4.af05slrtruoetgaxwwjv5nsq5e.px.internal.cloudapp.net>
Co-authored-by: Sergii Dymchenko <sedymche@microsoft.com>