[mlir][linalg] Add tests for tensor.unpack decomposition (#118786)

This commit adds additional tests and documentation for
`DecomposeOuterUnitDimsUnPackOpPattern` to ensure symmetry with its
counterpart for `tensor.pack`, `DecomposeOuterUnitDimsPackOpPattern`.

The new tests aim to improve implementation, documentation, and test
coverage for tensor.unpack. They cover the following scenarios:

* Static tile sizes: A simple `tensor.unpack` case
  (`@simple_unpack_static_tiles`).
* Dynamic tile size: `tensor.unpack` with a single dynamic tile size
  (`@simple_unpack_dynamic_tile`).
* Transpose: `tensor.unpack` with dynamic tile size and transpose
  (`@simple_unpack_dynamic_tile_transpose`), currently commented out due
  to some missing logic (see below)
* Scalable tile size: `tensor.unpack` with a scalable inner tile size
  (@simple_unpack_scalable_tile).

Notes:

The test `@simple_unpack_dynamic_tile_transpose` is commented out
because the logic for capturing dynamic sizes for `tensor::EmptyOp` when
some tile sizes are dynamic is incomplete. This missing functionality
will be addressed in a follow-up patch.

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