attention_ocr: Support TensorFlow Serving's Classification API

Prerequisites

Please answer the following question for yourself before submitting an issue.

• I checked to make sure that this feature has not been requested already.

1. The entire URL of the file you are using

https://github.com/tensorflow/models/blob/master/research/attention_ocr/python/model_export.py

2. Describe the feature you request

Add a Classification SignatureDef to the SavedModel, so that the model can be served through the Tensorflow Serving Classification API as described in https://stackoverflow.com/questions/63931981/error-when-serving-attention-ocr-model-error-expected-one-or-two-output-ten?sem=2

3. Additional context

The Classification API expects the output to contain 2 tensors, one for predictions and the other for scores. The existing code exports 17 tensors. The tensors that need to be exported for the classification API are endpoints.predicted_text and endpoints.normalized_seq_conf.

4. Are you willing to contribute it? (Yes or No)

yes