Update GradientModifier to be an in-place operation

[dxoigmn]

What does this PR do?

This PR updates GradientModifier to be an in-place operation like torch.nn.utils.clip_gradnorm. This also adds a test to make sure Adversary is properly modifying gradients.

Type of change

Please check all relevant options.

• [x] Improvement (non-breaking)
• [ ] Bug fix (non-breaking)
• [ ] New feature (non-breaking)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Testing

Please describe the tests that you ran to verify your changes. Consider listing any relevant details of your test configuration.

• [x] make test
• [x] CIFAR10_CNN_Adv achieves ~70% accuracy

Before submitting

• [x] The title is self-explanatory and the description concisely explains the PR
• [x] My PR does only one thing, instead of bundling different changes together
• [x] I list all the breaking changes introduced by this pull request
• [x] I have commented my code
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have run pre-commit hooks with pre-commit run -a command without errors

Did you have fun?

Make sure you had fun coding 🙃

[dxoigmn]

@mzweilin: I think it's better to merge this before the generic dispatch mechanism because it updates the tests.