[LITE][XPU] 1. Add precision switch(int16/int31) in XPUMultiEncoderOp; 2. Fix identity_dropout_eliminate_pass, |AttrType| of 'is_test' in OpDesc can be INT or BOOLEAN; 3. Enhance |__xpu__multi_encoder_fuse_pass|; !3596
Created by: Cwndmiao
- 增加XPUMultiEncoder中的精度切换，使用int16或int31；
- 修复identity_dropout_eliminate_pass中的属性类型问题，可以为INT或BOOLEAN；
- 把identity_dropout_eliminate_pass放到__xpu__multi_encoder_fuse_pass前面，__xpu__multi_encoder_fuse_pass能适配更多的场景；