Add TF Serving export script

[1vn]

Hi 👋

Thought this would be a useful script for people trying to serve this model on TF Serving. It reuses a lot of code from src/interactive_conditional_samples.py for loading the model into a live graph and then I used the TF SavedModel API to export it into a directory TF serving expects.

Usage:

python3 src/export_for_serving.py --top_k=40

Output:

tree models/124M/export
models/124M/export
└── 1566401179
    ├── saved_model.pb
    └── variables
        ├── variables.data-00000-of-00001
        └── variables.index