Using a local reference ensure that the Status object cannot be disposed before the Dispose. This way it's also possible to use an external Status instance instead of the static one, if needed.

* 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>

In multithreading .NET 4.8 applications, sometimes in Session finalizer the method c_api.TF_DeleteSession find f.Status static instance already disposed for some reason. No problem for .NET 6 application or with a single thread.