message: Fix docstring error of MlirPassthroughOp
  In api_def_MlirPassthroughOp.pbtxt, the docstring of MlirPassthroughOp has
  a typo (extra `=`) which needs to be removed:
  ```diff
  def foo(x, y):
  return = mlir_passthrough_op([x, y], mlir_module, Toutputs=[tf.float32])
  +  return mlir_passthrough_op([x, y], mlir_module, Toutputs=[tf.float32])
  ```
diff: @@ -27,7 +27,7 @@ func @main(%arg0 : tensor<10xf32>, %arg1 : tensor<10xf32>) -> tensor<10x10xf32>
  @tf.function
  def foo(x, y):
  - return = mlir_passthrough_op([x, y], mlir_module, Toutputs=[tf.float32])
  + return mlir_passthrough_op([x, y], mlir_module, Toutputs=[tf.float32])
  graph_def = foo.get_concrete_function(tf.TensorSpec([10], tf.float32), tf.TensorSpec([10], tf.float32)).graph.as_graph_def()
  ```
  