Add partial-width tile shape tests for hint-exercising paths in CCL and X collectives

[Copilot]

PR #432 added hint= parameters to iris.store() calls across CCL (all_gather, all_reduce, all_to_all) and iris/x/all_reduce.py. The hint path was only tested for x.all_gather (PR #435). This PR extends coverage to the remaining ops.

Changes

• tests/x/test_all_reduce.py: Added two new shapes to the existing parametrize list:

  • (128, 128, 64, 32) — BLOCK_N < N/world_size, exercises multi-block partial-width path
  • (256, 128, 64, 16) — BLOCK_N=16, exercises 16-bit vectorization path

• tests/ccl/test_all_gather.py: Upgraded (M, N) parametrize to (M, N, block_size_m, block_size_n) in both test_all_gather and test_all_gather_partitioned; block sizes forwarded via Config. Added the same two partial-width shapes.

• tests/ccl/test_all_reduce.py: Same restructure — block size params passed through Config(all_reduce_variant=..., block_size_m=..., block_size_n=...).

• tests/ccl/test_all_to_all.py: Same restructure — block sizes forwarded via Config.

The two new shapes added to each file:

(128, 128, 32, 32),  # BLOCK_N < N/world_size (partial-width, multi-block per rank)
(256, 128, 32, 16),  # Minimum BLOCK_N=16 (16-bit vectorization path)

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