Wrong sample_shape when name is given 

Wrong sample_shape when name is given and using size in the data model

```
import inferpy as inf
import tensorflow as tf

with inf.datamodel(size=100):
    x = inf.Normal(tf.ones(4), 1, name="x")

x.sample_shape  # []

with inf.datamodel(size=100):
    x = inf.Normal(tf.ones(4), 1)
    
    
x.sample_shape  # [100]
```