[vjp3] remove ad.backward_pass and non-fancy HOP transposes

[mattjj]

dead weight at this point

[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 represents a significant internal refactoring within JAX's automatic differentiation system, specifically targeting the vjp3 (third version of vector-Jacobian product) implementation. It removes the older ad.backward_pass and ad.closed_backward_pass functions, consolidating all primitive transpose rules under the more modern and flexible ad.fancy_transposes mechanism. This change streamlines the AD backend, eliminating redundant "non-fancy" transpose implementations and moving towards a unified approach that leverages backward_pass3 and ValAccum for improved gradient computation.

Highlights

• Deprecation of ad.backward_pass: The core backward_pass and closed_backward_pass functions, which were central to JAX's automatic differentiation, have been removed from their primary implementation and replaced with a compatibility shim that delegates to backward_pass3.
• Transition to fancy_transposes: All primitive transpose rules (for remat_p, custom_jvp_call_p, core.call_p, core.closed_call_p, cond_p, scan_p, jit_p) have been migrated from ad.primitive_transposes to ad.fancy_transposes, leveraging backward_pass3 and ValAccum for a more advanced VJP implementation.
• Cleanup of legacy transpose implementations: Several 'non-fancy' transpose functions and their associated helper functions (e.g., _custom_jvp_call_transpose, _transpose_cond_jaxpr, _scan_transpose, _pjit_transpose) have been entirely removed from the codebase, simplifying the AD backend.

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[yashk2810]

Woohoo

[gemini-code-assist]

Code Review

This pull request is a good step towards simplifying the automatic differentiation internals by removing the deprecated ad.backward_pass and non-fancy transpose rules. The changes are consistent with the goal of migrating to the vjp3 infrastructure. I've identified a potential bug in the updated _closed_call_transpose implementation and some leftover debugging code that should be removed.