Fix summary when optimizer is None

[mattdangerw]

Optimizer can be None (e.g. for just running inference). We should be careful not to assume it is always set when printing out model summary.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (beba6f1) 76.49% compared to head (060560c) 76.49%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #879 +/- ## ======================================= Coverage 76.49% 76.49% ======================================= Files 329 329 Lines 31334 31334 Branches 6100 6100 ======================================= Hits 23970 23970 Misses 5785 5785 Partials 1579 1579 ``` | [Flag](https://app.codecov.io/gh/keras-team/keras-core/pull/879/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | Coverage Δ | | |---|---|---| | [keras_core](https://app.codecov.io/gh/keras-team/keras-core/pull/879/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | `76.40% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/keras-team/keras-core/pull/879?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | Coverage Δ | | |---|---|---| | [keras\_core/utils/summary\_utils.py](https://app.codecov.io/gh/keras-team/keras-core/pull/879?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team#diff-a2VyYXNfY29yZS91dGlscy9zdW1tYXJ5X3V0aWxzLnB5) | `72.72% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[fchollet]

FAILED keras_core/utils/summary_utils_test.py::SummaryUtilsTest::test_print_model_summary0 - NotImplementedError: fit not implemented for NumPy backend.

We should exclude the Numpy backend in the test.