[converter] Import from an existing TFLite model and reuse the graph optimizer

Originated from the discussion in the DingTalk group. We will look into it if the demand is high. Currently, it is a low-priority task.

The issue names no files, tests, or entry points. Start by locating the converter and graph optimizer, then determine how an existing TFLite model should be imported and how optimizer reuse should work. Done means the agreed import and reuse behavior is implemented and covered by tests.

Written by the indexing model from the issue text.