Remove logic for appending or prepending tokens (#4920)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4920

The runner should not prepend or append any tokens to the prompt. Users should make sure that the prompt is properly formatted as the model's chat template.

This PR removes the logic for appending or prepending tokens.

Test Plan:
CI

imported-using-ghimport

Reviewed By: iseeyuan

Differential Revision: D61824091

Pulled By: helunwencser

fbshipit-source-id: f2f25c17dbcfb499e6865649dac8e020d9f5d802