Allow for graph visualization of media transformations

[wd60622]

I would like to see the graph associated with a transformation. A graphviz or to_graph method
would be the API.

It would also assume dummy dims for all required coords of a model.

from pymc_marketing.mmm import GeometricAdstock

adstock = GeometricAdstock(l_max=12)
G = adstock.to_graph()

The logic that is under the hood would be similar to that of sample_curve combined with calling
pm.model_to_graphviz

Some other examples:

from pymc_marketing.mmm import LogisticSaturation

saturation = LogisticSaturation()
G = adstock.to_graph()

from pymc_marketing.mmm import MediaTransformation

media_transformation = MediaTransformation(
    adstock=adstock, 
    saturation=saturation, 
    dims=(),
)
G = media_transformation.to_graph()

The adstock and saturation graph would be along media spend and media
transformation could be along date. Pretty easy to illustrate either way.