[samples/core/get_starter/eager]: Update with API simplifications in 1.8

* Add new import paths to mnist_tpu.py

This adds new import paths to mnist_tpu.py so individuals can check out the git repository and run `python mnist_tpu.py` without finagling with python import search paths.

* Fix lint errors

* Add disable= to pylint instructions

* Remove parent path

* Update comments

Pull doc fixes into version branch.

* "Machine Learning" --> Machine learning.
* "steepest the ascent" --> "steepest ascent"
* "standard gradient descent" --> "stochastic gradient descent"

Internal changes for object detection

First commit of learning_unsupervised_learning

Image encoder: fix binary files parsed as UTF-8

* Fix MNIST to allow master=local to work again.

* Update mnist_tpu.py

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- Fix externally broken test input_test.py
- Add Cython to object detection REQUIRED_PACKAGES

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Updating model to infer when labels are padded, rather than relying solely on the mode. This is necessary for evaluating on train data.

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Use keyword arguments in model builder for ssd feature extractor. This was incorreclty setting reuse argument.

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Removing flag defaults for train steps and eval steps. If not provided, will revert to the values specified in the config.

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Add warning if shuffle=false but num_readers > 1. The latter would cause the input data stream to still be shuffled.

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Cython is must installed package for building cocoapi. else the build will not
initiate.

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Tfmini requires positive values for the dimensions. Therefore, we need to test for no paddings in both width and height direction.

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Switch line orders in trainer so that restore_map is called after moving average variables are created.

Moving averages are now properly loaded during fine-tuning, instead of being recreated.

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