Fix wrong parsing of model config given on command line

[GPhilo]

Description

The TF2 version of the SSD TFLite model exporter allows to pass a --config_override flag with a TrainEvalPipelineConfig message to alter the given pipeline_config. However, the script calls Parse instead of Merge when handling the flags. This raises an error for any non-repeated items in the message. For example, using the value in the script's documentation, it raises:

google.protobuf.text_format.ParseError: 1:19 : Message type "object_detection.protos.TrainEvalPipelineConfig" should not have multiple "model" fields.

Simply replacing Parse with Merge (similarly to how the TF1 version of the script handles the flag) solves the problem.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Tests

Run the script with the command shown in the example usage on any pre-trained SSD models from the TF2 model zoo.

Checklist

• I have signed the Contributor License Agreement.
• I have read guidelines for pull request.
• My code follows the coding guidelines.
• I have performed a self code review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
  -> The change is minimal and self-explanatory
• I have made corresponding changes to the documentation.
  -> The fix makes the script work as already documented
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
  -> I'm not sure how to add a test for this (I'm not familiar with the testing infrastructure and I couldn't find any example for the script or its TF1 version)

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