I added `ToArray()` so that there is no "The collection has changed" error after `_delete_tracking`.

* Add CheckpointReader and corresponding C APIs.

* Add essential components of SavedModel format loading.

* Add checkpoint reading for SavedModel format loading.

* Revise customized json converters.

* Add support for loading models from python.

* Fix the duplicated weights in Keras.Model.

* Add alexnet loading test and check for loaded weights.

* Fix ci error caused by branch merge.

* Resolve the comments and errors.

* Fix the stucking of training when loading model.

* Fix the stucking of training when loading model.

* fix intptr.

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Co-authored-by: Haiping Chen <haiping008@gmail.com>

* Add check for dims of x and y in model.fit.

* Init the serialization of keras pb model.

* Add more facilities to the saved model framework.

* Add ListWrapper and ITrackable, and revise implmentations.

* Add serialized attributes.

* Implement layer serializations.

* Add lacked implementations (mainly MultiDeviceSaver).

* Support autograph.to_graph under graph mode.

* Add more implementations to the pb model save.

* Add more implementations to the keras part of pb model save.

* Refine some code after merge.

* Add two simple sequential test case of pb model save.

* Implement serializing attributes other keras arg definitions.

* Add alexnet pb save test.

* Check and refine the code.

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Co-authored-by: AsakusaRinne <AsakusaRinne@gmail.com>