Allow `ConstantOfShape` to take python constants

Instead of writing 

```python
op.ConstantOfShape(
    new_shape, value=make_tensor("zerof", TensorProto.FLOAT, [1], [0])
)
```

it would be great to be able to write

```python
op.ConstantOfShape(
    new_shape, value=0.0)
)
```

cc @gramalingam 