Enable calling the C API by default.

Enable calling the C API by default.

Enabling the C API makes TensorFlow's Python code to call the TensorFlow C API for low-level graph construction. This should cause no noticeable changes for the most part. One known difference is improved static shape inference, meaning some shape errors will be surfaced during graph construction instead of at runtime.

Note that this can be disabled by setting the environment variable TF_C_API_GRAPH_CONSTRUCTION=0 (for now, eventually this option will be removed).

PiperOrigin-RevId: 191536563