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Refactor object detection box predictors and fix some issues with model_main. (#4965)

* Merged commit includes the following changes:
206852642  by Zhichao Lu:

    Build the balanced_positive_negative_sampler in the model builder for FasterRCNN. Also adds an option to use the static implementation of the sampler.

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206803260  by Zhichao Lu:

    Fixes a misplaced argument in resnet fpn feature extractor.

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206682736  by Zhichao Lu:

    This CL modifies the SSD meta architecture to support both Slim-based and Keras-based box predictors, and begins preparation for Keras box predictor support in the other meta architectures.

    Concretely, this CL adds a new `KerasBoxPredictor` base class and makes the meta architectures appropriately call whichever box predictors they are using.

    We can switch the non-ssd meta architectures to fully support Keras box predictors once the Keras Convolutional Box Predictor CL is submitted.

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206669634  by Zhichao Lu:

    Adds an alternate method for balanced positive negative sampler using static shapes.

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206643278  by Zhichao Lu:

    This CL adds a Keras layer hyperparameter configuration object to the hyperparams_builder.

    It automatically converts from Slim layer hyperparameter configs to Keras layer hyperparameters. Namely, it:
    - Builds Keras initializers/regularizers instead of Slim ones
    - sets weights_regularizer/initializer to kernel_regularizer/initializer
    - converts batchnorm decay to momentum
    - converts Slim l2 regularizer weights to the equivalent Keras l2 weights

    This will be used in the conversion of object detection feature extractors & box predictors to newer Tensorflow APIs.

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206611681  by Zhichao Lu:

    Internal changes.

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206591619  by Zhichao Lu:

    Clip the to shape when the input tensors are larger than the expected padded static shape

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206517644  by Zhichao Lu:

    Make MultiscaleGridAnchorGenerator more consistent with MultipleGridAnchorGenerator.

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206415624  by Zhichao Lu:

    Make the hardcoded feature pyramid network (FPN) levels configurable for both SSD
    Resnet and SSD Mobilenet.

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206398204  by Zhichao Lu:

    This CL modifies the SSD meta architecture to support both Slim-based and Keras-based feature extractors.

    This allows us to begin the conversion of object detection to newer Tensorflow APIs.

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206213448  by Zhichao Lu:

    Adding a method to compute the expected classification loss by background/foreground weighting.

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206204232  by Zhichao Lu:

    Adding the keypoint head to the Mask RCNN pipeline.

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206200352  by Zhichao Lu:

    - Create Faster R-CNN target assigner in the model builder. This allows configuring matchers in Target assigner to use TPU compatible ops (tf.gather in this case) without any change in meta architecture.
    - As a +ve side effect of the refactoring, we can now re-use a single target assigner for all of second stage heads in Faster R-CNN.

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206178206  by Zhichao Lu:

    Force ssd feature extractor builder to use keyword arguments so values won't be passed to wrong arguments.

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206168297  by Zhichao Lu:

    Updating exporter to use freeze_graph.freeze_graph_with_def_protos rather than a homegrown version.

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206080748  by Zhichao Lu:

    Merge external contributions.

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206074460  by Zhichao Lu:

    Update to preprocessor to apply temperature and softmax to the multiclass scores on read.

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205960802  by Zhichao Lu:

    Fixing a bug in hierarchical label expansion script.

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205944686  by Zhichao Lu:

    Update exporter to support exporting quantized model.

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205912529  by Zhichao Lu:

    Add a two stage matcher to allow for thresholding by one criteria and then argmaxing on the other.

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205909017  by Zhichao Lu:

    Add test for grayscale image_resizer

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205892801  by Zhichao Lu:

    Add flag to decide whether to apply batch norm to conv layers of weight shared box predictor.

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205824449  by Zhichao Lu:

    make sure that by default mask rcnn box predictor predicts 2 stages.

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205730139  by Zhichao Lu:

    Updating warning message to be more explicit about variable size mismatch.

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205696992  by Zhichao Lu:

    Remove utils/ops.py's dependency on core/box_list_ops.py. This will allow re-using TPU compatible ops from utils/ops.py in core/box_list_ops.py.

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205696867  by Zhichao Lu:

    Refactoring mask rcnn predictor so have each head in a separate file.
    This CL lets us to add new heads more easily in the future to mask rcnn.

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205492073  by Zhichao Lu:

    Refactor R-FCN box predictor to be TPU compliant.

    - Change utils/ops.py:position_sensitive_crop_regions to operate on single image and set of boxes without `box_ind`
    - Add a batch version that operations on batches of images and batches of boxes.
    - Refactor R-FCN box predictor to use the batched version of position sensitive crop regions.

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205453567  by Zhichao Lu:

    Fix bug that cannot export inference graph when write_inference_graph flag is True.

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205316039  by Zhichao Lu:

    Changing input tensor name.

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205256307  by Zhichao Lu:

    Fix model zoo links for quantized model.

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205164432  by Zhichao Lu:

    Fixes eval error when label map contains non-ascii characters.

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205129842  by Zhichao Lu:

    Adds a option to clip the anchors to the window size without filtering the overlapped boxes in Faster-RCNN

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205094863  by Zhichao Lu:

    Update to label map util to allow the option of adding a background class and fill in gaps in the label map. Useful for using multiclass scores which require a complete label map with explicit background label.

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204989032  by Zhichao Lu:

    Add tf.prof support to exporter.

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204825267  by Zhichao Lu:

    Modify mask rcnn box predictor tests for TPU compatibility.

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204778749  by Zhichao Lu:

    Remove score filtering from postprocessing.py and rely on filtering logic in tf.image.non_max_suppression

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204775818  by Zhichao Lu:

    Python3 fixes for object_detection.

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204745920  by Zhichao Lu:

    Object Detection Dataset visualization tool (documentation).

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204686993  by Zhichao Lu:

    Internal changes.

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204559667  by Zhichao Lu:

    Refactor box_predictor.py into multiple files.
    The abstract base class remains in the object_detection/core, The other classes have moved to a separate file each in object_detection/predictors

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204552847  by Zhichao Lu:

    Update blog post link.

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204508028  by Zhichao Lu:

    Bump down the batch size to 1024 to be a bit more tolerant to OOM and double the number of iterations. This job still converges to 20.5 mAP in 3 hours.

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PiperOrigin-RevId: 206852642

* Add original post-processing back.