v0.9.12 Upsert and bug fixes

What's Changed

• fix: allow merge() with dataset as input by @wjones127 in #1869
• fix: small bug in _updated_centroids() in kmeans.py, wrong initialization of zero_counts by @lillianye in #1885
• docs: document the merge insert operation in python by @westonpace in #1892
• fix: handle null return from predicate evaluation in pushdown by @westonpace in #1898
• perf: speed up materialization when there is no block list by @westonpace in #1897
• fix: merge insert operation fails if any of the payload columns are non-nullable by @westonpace in #1899
• feat: enable composable and customizable sampler in PyTorch data loader by @eddyxu in #1900

Full Changelog: v0.9.11...v0.9.12