Memory leak in FuncGraph

[Esther2013]

There is memory leak when created a FuncGraph with resources in the graph.
When disposing the FuncGraph, the resources havn't been disposed never.

[Esther2013]

KerasHistory causes ResourceVariable not released.
_handle in placeholder is IntPtr.Zero.

[Oceania2018]

Same issue as #719.

[MPnoy]

As I understand it, because of this, when working in eager mode with Keras models, a memory leak occurs when model.Apply() or layer.Apply() is constantly called.