We currently have a basis_set keyword argument to qml.compile that specifies which operations to not decompose. If an operation is not in the basis set and does not have a decomposition, we leave it be.

We may sometimes want to decompose anything that can potentially be decomposed:

@partial(qml.compile, basis_set={})
@qml.qnode(qml.device('default.qubit'))
def circuit():
    qml.U2(0.5, 0.6, 0)
    return qml.expval(qml.Z(0))

The above syntax should indicate "as long as it has a decomposition, decompose it".

Right now we interpret {} as Falsy, and change the basis set to qml.ops.__all__. Instead, we should interpret {}, (), and [] (any non-None) object, as an actual basis set. An empty basis set should be a valid basis set.