Add TPU SavedModel exporter and refactor OD code (#6737)

247226201  by ronnyvotel:

    Updating the visualization tools to accept unique_ids for color coding.

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247067830  by Zhichao Lu:

    Add box_encodings_clip_range options for the convolutional box predictor (for TPU compatibility).

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246888475  by Zhichao Lu:

    Remove unused _update_eval_steps function.

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246163259  by lzc:

    Add a gather op that can handle ignore indices (which are "-1"s in this case).

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246084944  by Zhichao Lu:

    Keras based implementation for SSD + MobilenetV2 + FPN.

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245544227  by rathodv:

    Add batch_get_targets method to target assigner module to gather any groundtruth tensors based on the results of target assigner.

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245540854  by rathodv:

    Update target assigner to return match tensor instead of a match object.

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245434441  by Zhichao Lu:

    Add README for tpu_exporters package.

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245381834  by lzc:

    Internal change.

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245298983  by Zh...

research/object_detection/tpu_exporters/__init__.py

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