`VariableFactory` factory should come with a default `to_dict` and `from_dict` method

[williambdean]

The protocol defines the variable creation method only, but it should also define the to_dict and from_dict methods as well. It should maybe also define many other pieces of the signature like sample_prior, to_graph and create_likelihood_variable.

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This issue was transferred from pymc-labs/pymc-marketing#1359

Comments

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williambdean

Related to #1341