Add test_stack_different_required_keys

[ValerianRey]

At first, I wanted to make this test in test_stack.py, but actually, it depends on other transforms: we can't specify required keys with the FakeGradientsTransform defined in test_stack.py, and I think test files of autojac currently only depend on the transforms that they test, except for test_interactions.py which contains tests combining multiple transforms.
The alternative would have been to create another fake transform in test_stack.py, for instance one that would require some keys but that would not produce any output (kind of the opposite of the FakeGradientsTransform). I'm not sure it's worth the effort, and I like having this test apply on a real transform like Grad, so I prefer what I did.

[codecov]

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