Add parallel dispatch helper for CPU encoder

[varshneydevansh]

Proposed changes

• Adds a CommandEncoder::dispatch_parallel helper for CPU backend work.
  The helper schedules one ordered stream task, splits a [0, n) range into
  chunks, runs those chunks on a shared CPU thread pool, and waits for the
  futures before the stream task completes.

• This gives custom CPU primitive authors a built-in way to parallelize large
  kernel loops without managing their own thread pool.

• Fixes How to implement custom multi-thread cpu kernels? #2185

Tests: added TEST_CASE("test cpu dispatch parallel") in tests/scheduler_tests.cpp

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[varshneydevansh]

The default grain_size is 32768 is a conservative default that can be overridden :)

[varshneydevansh]

I kept here 4 as I found this in couple places -

mlx/mlx/io/load.cpp

Lines 337 to 347 in de7b4ed

	ThreadPool& thread_pool() {
	// Leak - see Scheduler singleton comment in scheduler.cpp.
	static ThreadPool* pool_ = new ThreadPool{4};
	return *pool_;
	}
	ThreadPool& ParallelFileReader::thread_pool() {
	// Leak - see Scheduler singleton comment in scheduler.cpp.
	static ThreadPool* thread_pool = new ThreadPool{4};
	return *thread_pool;
	}

and in this test file too int num_threads = 4;

[varshneydevansh]

This is just a fallback choice.

[varshneydevansh]

from my terminal -

devanshvarshney ❯❯ DEVICE=cpu ./build-cpu/tests/tests --test-case="test cpu dispatch parallel"
[doctest] doctest version is "2.4.12"
[doctest] run with "--help" for options
===============================================================================
[doctest] test cases:    1 |    1 passed | 0 failed | 243 skipped
[doctest] assertions: 1025 | 1025 passed | 0 failed |
[doctest] Status: SUCCESS!

devanshvarshney ❯❯ DEVICE=cpu ./build-cpu/tests/tests
[doctest] doctest version is "2.4.12"
[doctest] run with "--help" for options
===============================================================================
[doctest] test cases:  244 |  244 passed | 0 failed | 0 skipped
[doctest] assertions: 4253 | 4253 passed | 0 failed |
[doctest] Status: SUCCESS!

[zcbenz]

Thanks for the PR but we plan to merge the thread pool implementation in #3019, it is something that we wound't nave a good idea what the interface should look like without real world uses, so we don't want to add the API without actual multi-threaded cpu kernels, but still thanks anyway.

[varshneydevansh]

Gotcha.