For all current calculators, implement the option to return only metadata

[jwa7]

In the process of writing a new API for Clebsch Gordan iterations (see PR #237), it became apparent that generating pseudo-outputs of the CG calculation process (whereby TensorMaps with the correct metadata are returned but with no actual CG combinations are performed) is useful.

The input for public functions of the CG API is currently a TensorMap such as a rascaline.SphericalExpansion. Therefore, to calculate the metadata produced by CG iterations, one still needs to calculate the actual SphericalExpansion in the first place. Part of the point of pre-calculating metadata is cheaply see the size of descriptors generated with a given choice of hyperparameters, which isn't necessarily trivial to 'guess', especially when performing CG iterations.

It would therefore be useful if, for all rascaline calculators, one could calculate only the metadata. There may also be benefits when using jax, though with no expertise on this I'll let @Luthaf comment.

[Luthaf]

Couple of points: we already have code to do this in Rust: https://github.com/Luthaf/rascaline/blob/55ad1f14363812639c865801a6d517d8c5f363f6/rascaline/src/calculator.rs#L288.
We mainly need to expose this code to the C and then Python API.

Regarding jax, it looks like we need to be able to give the output shape of an operation without actually running it when defining custom primitives, cf https://jax.readthedocs.io/en/latest/notebooks/How_JAX_primitives_work.html#abstract-evaluation-rules, the "Abstract Evaluation Rules" section.