Skip tests if C2/ONNX models cannot be read (#18494)

Skip tests if C2/ONNX models cannot be read (#18494)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18494

Today we have some C2 end2end test run requiring reading model data from external filesystem (for example, Gluster and AWS). This could be a source for flaky test when the external filesystems are not reachable during the tests.

In this diff, we add try/catch logic around where we download models and open model files from external system. In case such attempts fails, we will catch the excption and let the unittest skip the current test instead of failure.

I also refactor the code a little bit by removing some duplicated logic on downloading and build the c2 model data. It has been duplicated in two classes and a few functions...

Reviewed By: yinghai

Differential Revision: D14442241

fbshipit-source-id: da8bf56c8d096efa34ca2070de5cd10a18aad70c