1. Add a new input named batch_roi_nums for prroi_pool_op. batch_roi_nums includes the number of roi for each image in batch when rois is Tensor. This information is saved in rois's lod when rois is LoDTensor.
2. add grad check to prroi_pool_op and solve unnormal X grad diff in CPU.

* fix the precise roi poop op test=develop
add roi backward implementation, fix the output-channel

* add precise roi pooling op test=develop

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* detail the description test=develop

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* elaborate the doc for return type test=develop

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* Add psroi_pool operator.