chore: use rayon in pq transform and remove async calls

[eddyxu]

one nit + would you happen to have benchmark numbers? did not find a handy benchmark for PQ::transform yet, could be next time.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.33962% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 80.79%. Comparing base (4cb0a2b) to head (0e374af).

Files	Patch %	Lines
rust/lance-linalg/src/kmeans.rs	81.81%	4 Missing :warning:
rust/lance-index/src/vector/pq.rs	98.76%	0 Missing and 1 partial :warning:
rust/lance-index/src/vector/pq/transform.rs	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2343 +/- ## ========================================== + Coverage 80.78% 80.79% +0.01% ========================================== Files 192 192 Lines 56189 56213 +24 Branches 56189 56213 +24 ========================================== + Hits 45392 45420 +28 - Misses 8128 8129 +1 + Partials 2669 2664 -5 ``` | [Flag](https://app.codecov.io/gh/lancedb/lance/pull/2343/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lancedb) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/lancedb/lance/pull/2343/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lancedb) | `80.79% <94.33%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lancedb#carryforward-flags-in-the-pull-request-comment) to find out more.

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