How to use the pyhf.set_backend function in pyhf

pdf = pyhf.Model(spec)

    data = source['bindata']['data'] + pdf.config.auxdata

    backends = [
        pyhf.tensor.numpy_backend(),
        pyhf.tensor.tensorflow_backend(session=tf.compat.v1.Session()),
        pyhf.tensor.pytorch_backend(),
    ]

    test_statistic = []
    for backend in backends:
        if backend.name == 'tensorflow':
            tf.reset_default_graph()
            backend.session = tf.compat.v1.Session()
        pyhf.set_backend(backend)

        q_mu = pyhf.infer.hypotest(
            1.0,
            data,
            pdf,
            pdf.config.suggested_init(),
            pdf.config.suggested_bounds(),
            return_test_statistics=True,
        )[-1][0]
        test_statistic.append(pyhf.tensorlib.tolist(q_mu))

    # compare to NumPy/SciPy
    test_statistic = np.array(test_statistic)
    numpy_ratio = np.divide(test_statistic, test_statistic[0])
    numpy_ratio_delta_unity = np.absolute(np.subtract(numpy_ratio, 1))

def test_set_backend(backend):
    tb, _ = pyhf.get_backend()
    pyhf.set_backend(tb.name)

def test_custom_backend_name_supported():
    class custom_backend(object):
        def __init__(self, **kwargs):
            self.name = "pytorch"

    with pytest.raises(AttributeError):
        pyhf.set_backend(custom_backend())

def test_custom_backend_name_notsupported():
    class custom_backend(object):
        def __init__(self, **kwargs):
            self.name = "notsupported"

    backend = custom_backend()
    assert pyhf.tensorlib.name != backend.name
    pyhf.set_backend(backend)
    assert pyhf.tensorlib.name == backend.name

def test_tensorflow_tolist_nosession():
    pyhf.set_backend(pyhf.tensor.tensorflow_backend())
    tb = pyhf.tensorlib

    # this isn't covered by test_list_to_list since we need to check if it's ok
    # without a session explicitly
    assert tb.tolist([1, 2, 3, 4]) == [1, 2, 3, 4]
    with pytest.raises(RuntimeError):
        # but a tensor shouldn't
        assert tb.tolist(tb.astensor([1, 2, 3, 4])) == [1, 2, 3, 4]

def test_supported_backends():
    pyhf.set_backend("fail")

def test_set_backend_by_string(backend_name):
    pyhf.set_backend(backend_name)
    assert isinstance(
        pyhf.tensorlib,
        getattr(pyhf.tensor, "{0:s}_backend".format(backend_name.lower())),
    )

def test_set_backend_by_bytestring(backend_name):
    pyhf.set_backend(backend_name)
    assert isinstance(
        pyhf.tensorlib,
        getattr(pyhf.tensor, "{0:s}_backend".format(backend_name.decode("utf-8"))),
    )

def test_supported_backends(backend_name):
    with pytest.raises(pyhf.exceptions.InvalidBackend):
        pyhf.set_backend(backend_name)