improve nondiff argnums error when a tracer is passed

[mattjj]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @mattjj, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the error handling within JAX's custom derivatives by introducing a robust mechanism to detect and report the incorrect usage of Tracer objects with nondiff_argnums. The change ensures that users receive immediate and clear feedback when attempting to mark a differentiable argument (represented by a Tracer) as non-differentiable, thereby improving the API's predictability and user experience.

Highlights

• Improved Error Handling: The system now explicitly raises an error when a JAX Tracer object is passed to an argument specified in nondiff_argnums, preventing silent misbehavior.
• Detailed Error Messages: The new error message provides precise information, including the argument index and the specific path within the argument where the Tracer was detected, aiding in quicker debugging.
• Refactored Tracer Validation: The previous _stop_gradient utility has been replaced with dedicated _check_tracer functions, allowing for more targeted and informative validation of nondiff_argnums.

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[gemini-code-assist]

Code Review

This pull request improves the error message when a JAX tracer is passed as a non-differentiable argument to custom_jvp. Instead of silently stopping the gradient, the new implementation raises an exception with a detailed message, including the argument index and path within the pytree. This is a great improvement for user experience and debugging. I have one suggestion to make the error message even more helpful and to use a more specific exception type.

[gemini-code-assist]

This is a great improvement for error reporting! To make it even better, consider using the more specific UnexpectedTracerError (which is already imported) instead of the generic Exception. Also, the error message could be more descriptive to help users understand why the error is happening and how to fix it, similar to the error message in _check_for_tracers for custom_vjp. I've also taken the liberty to rename pathstr to path_str for PEP8 compliance.

Suggested change

	pathstr = f' path {keystr(path)}' if path else ''
	raise Exception(f"Tracer of type {x.aval} passed at argument {i}{pathstr}, "
	"but Tracers can't be passed as nondiff_argnums.")
	path_str = f' at path {keystr(path)}' if path else ''
	msg = (f"A JAX Tracer of type {x.aval} was passed to a custom_jvp function "
	f"at argument {i}{path_str}, which was specified as non-differentiable. "
	"Tracers cannot be passed as non-differentiable arguments. Instead, "
	"`nondiff_argnums` should only be used for arguments that are not JAX "
	"tracers, e.g. Python scalars, strings, or callables.")
	raise UnexpectedTracerError(msg)