fix fetch op rename_input bug in QAT export model

[yghstill]

PR types

Bug fixes

PR changes

APIs

Describe

If next_op is fetch and out_var_name in fetch_targets, fetch_targets must update, so fix fetch op rename_input bug in QAT export model.

[paddle-bot-old]

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[wanghaoshuang]

conv2d_1-->tensor_A-->moving_average_abs_max_scale-->tensor_B-->conv2d_2
                                                        |
                                                         --->fetch

To remove unused moving_average_abs_max_scale from quantization graph, the input of conv2d_2 will be renamed to tensor_A. And hence tensor_B will be pruned with moving_average_abs_max_scale when saving inference model.
So we should replace tensor_B with tensor_A in fetched list.