[mlir] Fix warnings

This patch fixes:

  mlir/lib/Dialect/Mesh/Transforms/ShardingPropagation.cpp:73:27:
  error: unused function 'operator<<' [-Werror,-Wunused-function]

  mlir/lib/Dialect/Mesh/Transforms/ShardingPropagation.cpp:97:27:
  error: unused function 'operator<<' [-Werror,-Wunused-function]
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