> Torch, Jax

Both of these "make it available". Just because people don't know how to use/find them doesn't mean they're not "available".

> Eigen

This is not an ML anything, it's a linear algebra library.

> like commenting '# (b, n, t)' on every line, or suffixing shapes to variable names

There's a difference between tracking shapes in the compiler and specifying shapes in the model.