chore: define shared atol semantics across attacks

[shamykyzer]

Jim asked on #431 issue for the atol=1e-8 in InstanceBasedAttack to be globally defined, alongside whatever StructuralAttack uses for equivalence classes

InstanceBasedAttack uses np.allclose(atol=1e-8), StructuralAttack uses np.unique(axis=0) exact equality, so before extracting a constant I wanted to check whether you would like one shared tolerance across attacks, or two distinct constants with different semantics

@rpreen what do you think

[shamykyzer]

Hi @rpreen, could you please get back to me on this if you have time? Thanks a lot.

[rpreen]

I didn't work on the StructuralAttack so I'm not really sure, but I would imagine if it uses a constant it should be its own variable so it can be set separately from other attacks like InstanceBasedAttack? In other words: why should they share the same value?