`LookupTable`.
    *   Fix bug where `vectorized_map` failed on inputs with unknown static
        shape.
    *   Add preliminary support for sparse CSR matrices.
    *   Tensor equality with `None` now behaves as expected.
    *   Make calls to `tf.function(f)()`, `tf.function(f).get_concrete_function`
        and `tf.function(f).get_initialization_function` thread-safe.