Add a loop test for onnxified net (#32935)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32935
Mock away the content of onnxified net with some low cost ops so that we can still mimic the input/output transfer while doing minimal work on the card.
Test Plan:
```
buck run glow/fb/test:sparsenn_test -- --gtest_filter='SparseNNTest.vanillaC2' --onnxifi_debug_mode --onnxifi_loop_test_mode --nocaffe2_predictor_use_memonger
```
Differential Revision: D19631971
fbshipit-source-id: f970c55ccb410702f479255eeb750e01e3f8c2ae
