Opt-in feature

[AzHicham]

In order to reduce compile time / complexity.
It could be nice to make tch-rs and ndarray option feature.
WDYT ?

[Tudyx]

Good idea 👍
tch-rs is already under a features flag as it is optional. We should move it to an explicit features as it will become the default for the 2024 edition.
We can also feature gate ndarray for sure.

We would then have to choose a default back-end then, using the default feature.

What do you use the most, ndarray or tch-rs?