Add foundation_path to chemprop input for users to specify pretraining weight files

[dwwest]

Description

This will add a way to specify pretrained weights for chemprop

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• Manual Verification: I have manually reviewed and tested the code in this PR.
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• Scoped Change: This PR addresses a single, well-scoped issue rather than multiple unrelated changes.

Status

• Ready to go (Checking this signals to maintainers that the PR is ready for final review)

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 20.83333% with 19 lines in your changes missing coverage. Please review.

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[hmacdope]

Needs a test I think