Add unit tests for PGD implementation in TF2

[AlexeyKurakin]

No description provided.

[WindQAQ]

Hi @AlexeyKurakin, is anyone working on this? If not, I would like to do this. Thanks!

[npapernot]

No one is working on TF2 tests, so feel free to take it! Thanks! You can look at #1045 for some guidance.

[WindQAQ]

Sure! Want to make sure something here:

1. do we need to decorate the tests with run_all_in_graph_and_eager_modes to make them compatible with graph mode?
2. should we inherit the tests from tf.test.TestCase instead of CleverHansTest? It overrides some methods to make unittest more suitable for tensor.
   Thanks!

[npapernot]

1. Let me get back to you on this one
2. Yes that sounds like a good idea if it simplifies your implementation

[WindQAQ]

Thanks you! I would create a PR for fast_gradient_method first to let you see if it is good enough.

[npapernot]

Regarding #1, we had a discussion with @AlexeyKurakin and our current stand is that we should aim to verify the possibility of wrapping all attack functions in @tf.function in TF2 but that we do not need to verify compatibility with TF1 graph mode.

[WindQAQ]

Regarding #1, we had a discussion with @AlexeyKurakin and our current stand is that we should aim to verify the possibility of wrapping all attack functions in @tf.function in TF2 but that we do not need to verify compatibility with TF1 graph mode.

Okay! I'm also fine with this.