[refactor] Get rid of the torchfl.models.core subpackage and related updates

[vivekkhimani]

Once we have a stable YAML support, we can get rid of the torchfl.models.core subpackage as we don't need to code-generate those repetitive files. The objects generated by those files can be achieved via YAML configs for the models.

Acceptance Criteria:

• Get rid of the torchfl.models.core subpackage and the related reference from torchfl.* codebase.
• Update the examples to not use those references. If anyone's willing to achieve the same functionality from the Python script, they can import a model from torchfl.sota and get it done.
• Update the documentation:
• • README.md
• • modules information within the sphinx auto-generated docs.