Added clearer error message for tracers in numpy.split (#2508)
* Added clearer error message for tracers in numpy.split
Now we print:
ConcretizationTypeError: Abstract tracer value where concrete value is expected (in
jax.numpy.split argument 1).
Use transformation parameters such as `static_argnums` for `jit` to avoid
tracing input values.
See `https://jax.readthedocs.io/en/latest/faq.html#abstract-tracer-value-where-concrete-value-is-expected-error`.
Encountered value: Traced<ShapedArray>
* Fixed tests, slight change to the error message
* Expanded the FAQ entry about abstract tracers for higher-order primitives
* Added clarification for tracers inside jit of grad
* Updated FAQ language in response to reviews
