Add ability to load in pretraining weights for finetuning in chemprop

[dwwest]

We want to be able to load weights from pretraining from external sources.

This PR will affect only chemprop, but maybe this could eventually be extended to other foundation models.

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[hmacdope]

Does this not already exist @smcolby or am I confused.

[smcolby]

Current load path uses deserialize, so we need paths to the parameters and weights. Devany might be referring to a situation where there's just the weights not trained by us?