Revert "Parallelise perm_aggreg sub arrays"

[dannywillems]

This reverts commit 3647beb.

For the context, see MinaProtocol/mina#16782 (comment). The test in Mina is failing with the following error at this exact commit:

File "src/lib/pickles/test/chunked_circuits/dune", line 2, characters 8-15:
2 |  (names chunks2 chunks4 chunks8)
            ^^^^^^^
Uncaught exception:
  (monitor.ml.Error
   (Failure "Prover(\"rest of division by vanishing polynomial\")")
   ("Raised at Base__Result.ok_exn in file \"src/result.ml\", line 201, characters 17-26"
    "Called from Async_kernel__Deferred1.M.map.(fun) in file \"src/deferred1.ml\", line 17, characters 40-45"
    "Called from Async_kernel__Job_queue.run_job in file \"src/job_queue.ml\" (inlined), line 128, characters 2-5"
    "Called from Async_kernel__Job_queue.run_jobs in file \"src/job_queue.ml\", line 169, characters 6-47"
    "Caught by monitor block_on_async"))
Raised at Base__Result.ok_exn in file "src/result.ml" (inlined), line 201, characters 17-26
Called from Async_unix__Thread_safe.block_on_async_exn in file "src/thread_safe.ml", line 132, characters 29-63
Called from Async_unix__Thread_safe.block_on_async_exn in file "src/thread_safe.ml" (inlined), line 181, characters 27-54
Called from Dune__exe__Chunks2.test.test_prove in file "src/lib/pickles/test/chunked_circuits/chunks2.ml", line 116, characters 6-63
Called from Dune__exe__Chunks2 in file "src/lib/pickles/test/chunked_circuits/chunks2.ml", line 132, characters 2-9
File "src/lib/pickles/test/chunked_circuits/dune", line 2, characters 16-23:
2 |  (names chunks2 chunks4 chunks8)
                    ^^^^^^^
compile time: 0.100414s
Uncaught exception:
  (monitor.ml.Error
   (Failure "Prover(\"rest of division by vanishing polynomial\")")
   ("Raised at Base__Result.ok_exn in file \"src/result.ml\", line 201, characters 17-26"
    "Called from Async_kernel__Deferred1.M.map.(fun) in file \"src/deferred1.ml\", line 17, characters 40-45"
    "Called from Async_kernel__Job_queue.run_job in file \"src/job_queue.ml\" (inlined), line 128, characters 2-5"
    "Called from Async_kernel__Job_queue.run_jobs in file \"src/job_queue.ml\", line 169, characters 6-47"
    "Caught by monitor block_on_async"))
Raised at Base__Result.ok_exn in file "src/result.ml" (inlined), line 201, characters 17-26
Called from Async_unix__Thread_safe.block_on_async_exn in file "src/thread_safe.ml", line 132, characters 29-63
Called from Async_unix__Thread_safe.block_on_async_exn in file "src/thread_safe.ml" (inlined), line 181, characters 27-54
Called from Dune__exe__Chunks4.test.test_prove in file "src/lib/pickles/test/chunked_circuits/chunks4.ml", line 66, characters 6-63
Called from Dune__exe__Chunks4 in file "src/lib/pickles/test/chunked_circuits/chunks4.ml", line 77, characters 2-9
File "src/lib/pickles/test/chunked_circuits/dune", line 2, characters 24-31:
2 |  (names chunks2 chunks4 chunks8)
                            ^^^^^^^
compile time: 0.100318s
Uncaught exception:
  (monitor.ml.Error
   (Failure "Prover(\"rest of division by vanishing polynomial\")")
   ("Raised at Base__Result.ok_exn in file \"src/result.ml\", line 201, characters 17-26"
    "Called from Async_kernel__Deferred1.M.map.(fun) in file \"src/deferred1.ml\", line 17, characters 40-45"
    "Called from Async_kernel__Job_queue.run_job in file \"src/job_queue.ml\" (inlined), line 128, characters 2-5"
    "Called from Async_kernel__Job_queue.run_jobs in file \"src/job_queue.ml\", line 169, characters 6-47"
    "Caught by monitor block_on_async"))
Raised at Base__Result.ok_exn in file "src/result.ml" (inlined), line 201, characters 17-26
Called from Async_unix__Thread_safe.block_on_async_exn in file "src/thread_safe.ml", line 132, characters 29-63
Called from Async_unix__Thread_safe.block_on_async_exn in file "src/thread_safe.ml" (inlined), line 181, characters 27-54
Called from Dune__exe__Chunks8.test.test_prove in file "src/lib/pickles/test/chunked_circuits/chunks8.ml", line 66, characters 6-63
Called from Dune__exe__Chunks8 in file "src/lib/pickles/test/chunked_circuits/chunks8.ml", line 77, characters 2-9

[Copilot]

Pull Request Overview

This pull request reverts the previous commit "Parallelise perm_aggreg sub arrays" by restoring sequential computation for permutation aggregation.

• Replaces the parallel computation (using par_iter and reduce_with) with sequential loops.
• Removes the z_prefolded vector computation and inlines the corresponding product accumulation.