Make layer method `compute_output_shape` public.

Motivations:
- Useful for computing the shape of a layer's output without calling the layer.
- It is public in standalone keras (hence API discrepancy, which is something to be avoided).
- With eager mode and deferred mode for Network building, it is going to be increasingly necessary for users to implement this method or call it.
- Lots of internal users are apparently already relying on it, which highlights the importance of making this feature publicly available.
PiperOrigin-RevId: 180854139