Make the test for "unused inputs" more precise in `tf.import_graph_def()`.

Before, `tf.import_graph_def()` would raise a `ValueError` if any of
the tensors named in the `input_map` was not used as an input to
another node. However, the contract for this function states that a
`ValueError` will be raised "if `input_map`... contains names that do
not appear in `graph_def`," so this change expands the valid domain of
`input_map` to include tensors that only appear as unconsumed
operation outputs in the imported graph.

PiperOrigin-RevId: 161826633