Support to create a gradient machine with merged model in c-api

[Xreki]

Add a c-api interface,

/**
 * @brief Create a gradient machine used for model inference, using config with
 *        parameters which is generated by `paddle merge_model`.
 * @param [out] machine that used for model inference.
 * @param [in] mergedModel
 * @param [in] size
 * @return paddle_error
 */
PD_API paddle_error
paddle_gradient_machine_create_for_inference_with_parameters(
    paddle_gradient_machine* machine, void* mergedModel, uint64_t size);

where mergedModel is generated by paddle merge_model, and it contains config of network and all parameters.