Refactor input pipeline with synthetic data iterator

[NuojCheng]

Description

TL;DR

Re-organize input_pipeline_interphase.py for better readability

Following changes are made:

• Synthetic data iterator and placeholder synthetic data iterator are moved to a separate file
• BadSyntheticDataIterator is renamed to PlaceHolderDataIterator
• The if-else commands in the end are re-organized

FIXES: b/421596013

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

Checklist

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• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.