Creates an extractor for extracting example weights.

The extractor's PTransform uses the config's ModelSpec.example_weight_key(s) to lookup the associated example weight values stored as features under the tfma.FEATURES_KEY (and optionally tfma.TRANSFORMED_FEATURES_KEY) in extracts. The resulting values are then added to the extracts under the key tfma.EXAMPLE_WEIGHTS_KEY.

eval_config Eval config.

Extractor for extracting example weights.