[Relay] Improve frontend testing infrastructure.

[jroesch]

There has been a recent number of issues found in various frontends for Relay. I propose to ensure robustness and compatibility with popular model architectures we make an effort to import commonly used models and test their correctness.

We should consider more complex scenarios such as PyTorch -> ONNX -> Relay. I have added some initial tests in #3084 but there are still some issues in supporting all of torchvision.

[ethanjyx]

@jroesch @tqchen is anyone taking this? if not, i'm interested in taking this. I'm interested in making more contributions to tvm in the longer run, this testing infrastructure could be a good start for me to understand some of the code :)

[tqchen]

close for now, the testcases has been improving and it would be good to open new smaller items that are actionable