test_jit.py: Speedup EndToEnd tests by reducing workload size. (#15906)

test_jit.py: Speedup EndToEnd tests by reducing workload size. (#15906)

Summary:
Currently these tests are taking most of the time in test_jit.py run, with the
proposed changes the testing time is reduced by ~75%:

```
TestEndToEndHybridFrontendModels.test_neural_style: 203.360s -> 10.650s
TestEndToEndHybridFrontendModels.test_snli: 422.315s -> 9.152s
TestEndToEndHybridFrontendModels.test_super_resolution: 73.362s -> 19.185s

time python test/test_jit.py (real): 13m50.828s -> 3m11.768s
time python test/test_jit.py (user): 85m59.745s -> 13m18.135s
time python test/test_jit.py (sys): 144m9.028s -> 25m58.019s
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15906

Differential Revision: D13619659

Pulled By: ZolotukhinM

fbshipit-source-id: 6c22d8740f8ddb865c3a0667af32653723383816