Get rid of dummy type input to RVs

[ricardoV94]

Description

RandomVariables third input is a numerical integer than indexes into a list of predefined types to indicate if a variable is float64 or float32, ...

This input is useless because PyTensor variables are not allowed to change types at runtime. We should make it a property of the Op. This requires defining Ops dynamically whenever they are initialized, which means there is no canonical pt.random.normal. This is not too bad imo and simplifies handling (and visualizing) RV graphs