[Unity][Transform] Handle dynamic shapes in CombineParallelMatmul

[Lunderberg]

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Prior to this commit, if the weight of a matmul a dynamic shape, and that matmul is being combined with the CombineParallelMatmul pass, it could cause a segfault when dim.as<IntImmNode>() returns a null pointer.

This commit adds explicit test cases for these dynamic shapes, and updates CombineParallelMatmul to handle the dynamic shapes.