[Experimental] Fixing Tinker rollout prompt ids

[listar2000]

What does this PR do?

It does the following fixes to the experimental/rollout/tinker_engine:

• Make sure that the EncodedTextChunk is flattened to list[int] so that the prompt_ids in ModelOutput will consist of either int or ImageChunk (when using Tinker.renderer to tokenize messages).
• We also change the common visualization code to skip any non-integer element in prompt_ids.

Additionally, it also adds a few features to the experimental/rollout/completer (a wrapper feature on top of rollout_engine that directly returns Step):

• Add a reset() method to all completers.
• Add a counter to return prefix (accumulation) metrics for the TITOCompleter.

All the changes above are targetting experimental features. So these changes are non-breaking.