Create a TF_Graph alongside the Python graph.

This is a first step towards porting the Python API to use the C API. As the Python Graph and Operations are constructed, an analogous TF_Graph and TF_Operations are created via SWIG. Currently nothing is done with the TF_Graph; a next step will be switching to the new TF_Session API which runs a TF_Graph directly (instead of a GraphDef).

This new functionality is disabled by default and can be manually enabled by setting the _USE_C_API global in ops.py. For this patch I only enabled it for a single test file. I tried enabling it for all TF Python tests and manually disabling it for unsupported tests, but there were too many failing tests (although most tests passed). See ops.py for a TODO list of unsupported functionality.

I benchmarked building an Inception model, and building the TF_Graph incurs a 20% overhead to the total graph construction time. Note that this patch does not remove any existing Python functionality; another next step will be recovering this time by removing redundant Python code. There is no measurable overhead with the new functionality disabled.

PiperOrigin-RevId: 155655064

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