Fix bugs for updating from tf 2.0b0 to rc0
Fix bugs for updating from tf 2.0b0 to rc0

Updating the dependency of tensorflow from 2.0.0b1 to 2.0.0rc0 is causing crashes in keras-tuner.
Not sure the "set" operation is really useful, but it is causing the crash because it tries to hash a tensor with equality enabled.

```
../../.virtualenvs/ak/lib/python3.6/site-packages/kerastuner/utils.py:23: in compute_model_size
    params = [K.count_params(p) for p in set(model.trainable_weights)]
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <tf.Variable 'dense/kernel:0' shape=(32, 32) dtype=float32, numpy=
array([[ 0.03147817, -0.30117437, -0.08552131, ...,...],
       [ 0.23273078, -0.10847142,  0.0151484 , ...,  0.16468436,
        -0.21210298, -0.1087181 ]], dtype=float32)>

    def __hash__(self):
      if ops.Tensor._USE_EQUALITY and ops.executing_eagerly_outside_functions():  # pylint: disable=protected-access
>       raise TypeError("Variable is unhashable if Tensor equality is enabled. "
                        "Instead, use tensor.experimental_ref() as the key.")
E       TypeError: Variable is unhashable if Tensor equality is enabled. Instead, use tensor.experimental_ref() as the key.

../../.virtualenvs/ak/lib/python3.6/site-packages/tensorflow_core/python/ops/variables.py:1085: TypeError
```
keras-team/keras-tuner,keras-team/keras-tuner