Support empty inputs into executorch method (#1765)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1765

As titled. With constant method support, we are able to serialize a constant method that returns a constant without any input. In our python binding we want to support this feature.

Reviewed By: mikekgfb

Differential Revision: D53201972

fbshipit-source-id: 5826a118d84109441ee677272b76a33aed08687c