Don't show optimizer params if not optimizer or not built

[mattdangerw]

Most people will call summary() before fit(). Doing so will usually mean the optimizer would show up as 0 parameters, which is misleading.

[mattdangerw]

Alternately, we could show a (unbuilt) status.

Model: "res_net_2"
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
┃ Layer (type)                               ┃ Output Shape                        ┃       Param # ┃
┡━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
│ dense_6 (Dense)                            │ ?                                   │   0 (unbuilt) │
├────────────────────────────────────────────┼─────────────────────────────────────┼───────────────┤
│ dense_7 (Dense)                            │ ?                                   │   0 (unbuilt) │
├────────────────────────────────────────────┼─────────────────────────────────────┼───────────────┤
│ dense_8 (Dense)                            │ ?                                   │   0 (unbuilt) │
└────────────────────────────────────────────┴─────────────────────────────────────┴───────────────┘
 Total params: 0 (0.00 B)
 Trainable params: 0 (0.00 B)
 Non-trainable params: 0 (0.00 B)
 Optimizer params: 0 (0.00 B) (unbuilt)

In some ways this is more consistent maybe? But would also mean most people will see an unbuilt optimizer in the standard usage and not be sure how to build it.

Seemed potentially least confusing to just leave it off, but open to suggestions!

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.06% :tada:

Comparison is base (bb21710) 76.49% compared to head (a4a78a3) 76.56%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #885 +/- ## ========================================== + Coverage 76.49% 76.56% +0.06% ========================================== Files 329 329 Lines 31334 31425 +91 Branches 6100 6114 +14 ========================================== + Hits 23970 24060 +90 - Misses 5785 5786 +1 Partials 1579 1579 ``` | [Flag](https://app.codecov.io/gh/keras-team/keras-core/pull/885/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/885/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | `76.46% <100.00%> (+0.06%)` | :arrow_up: | 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/885?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/885?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team#diff-a2VyYXNfY29yZS91dGlscy9zdW1tYXJ5X3V0aWxzLnB5) | `73.11% <100.00%> (+0.38%)` | :arrow_up: | ... and [8 files with indirect coverage changes](https://app.codecov.io/gh/keras-team/keras-core/pull/885/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team)

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