Fetch C shapes for ops created by import_graph_def with C API enabled.

Fetch C shapes for ops created by import_graph_def with C API enabled.

If _USE_C_API = True, this change makes us always fetch shapes using
the C API after calling TF_ImportGraphDef, even if _USE_C_SHAPES =
False. This is necessary to preserve the shapes specified by the
"_output_shapes" attr on imported NodeDefs (note that this attr isn't
present on the NodeDefs of the imported nodes, so there's no other way
to recover this information after calling TF_ImportGraphDef).

PiperOrigin-RevId: 190122991