Made VGGish and YAMNet work in TF2 without disabling TF2 behavior.

[plakal]

Allowed TF2 behavior and allowed passing in a features tensor into the
VGGish model definition. Both of these changes are needed for making
TF-Hub exports of these models. Lifted constraints on TF versions since
tf_slim has been updated to work with TF 2.

Description

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Type of change

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• Documentation update
• TensorFlow 2 migration

Tests

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Test Configuration:

VGGish: Ran vggish_smoke_test.py and vggish_inference_demo.py with latest TF1 and TF2
YAMNet: Only documentation change.

Checklist

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• I have performed a self code review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.