(WIP) Feature: fix ENABLE_MULTI_MODEL setting

[taylorfturner]

Issue #, if available: N/A

Description of changes: Resolving how ENABLE_MULTI_MODEL is set in the code.

Testing done:

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[HammySession]

Found this code, affects deployment of pytorch_huggingface_inference, by never allowing the new handler to run and only using the default handler unless SAGEMAKER_MULTI_MODEL is set to False.