Use WeakKeyDictionaries for global Keras {graph->...} maps

These globals were holding onto graphs including FuncGraphs, which
held onto captured tensors leaving garbage around.

This change also adds a test to catch garbage like this in the future.
To make the test work, I needed to manually breakup some reference
cycles caused by OrderedDicts. We should probably have a custom impl
of OrderedDict similar to the one in Python3 and avoid these issues.

PiperOrigin-RevId: 212694290