About some problems of C-API

[hedaoyuan]

• Have exception for interface calls
  • Crashes due to multiple calls to paddle_init.[Fixd by Fix crashes due to multiple calls to paddle_init #5679]
  • When the input is empty, paddle_matrix_get_shape interface call will be abnormal.
• Memory leak problem
  • Currently, the memory is not freed after the paddle_gradient_machine_destroy interface is called.[Fix by The pool_limit_size need be zero in mobile inference. #5711]
• How to load multiple different models to make the inference.
• How to batch computing for multiple samples.