Wrap sparse operation in Lambda in tests (#1750)
Wrap sparse operation in Lambda in tests (#1750)

TensorFlow Keras generally requires that sparse operations happen within a
layer, not just as operations directly on the raw tensors. This validation
apparently previously missed the `tf.sparse.expand_dims` operation, but it is
now caught in TF 2.3.0-rc0 (see #1742, in particular
https://github.com/stellargraph/stellargraph/runs/817093240#step:6:90).

This PR pre-emptively fixes the test, so that we do not have to diagnose or
debug it in future.

Test run of this PR using TF 2.3.0-rc0, which passes:
https://github.com/stellargraph/stellargraph/pull/1750/checks?check_run_id=817247674

See: #1749

stellargraph/stellargraph,stellargraph/stellargraph