[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)

* [LITE][XPU] Add precision switch(int16/int31) in XPUMultiEncoderOp

* [LITE][XPU] fix identity_dropout_eliminate_pass, |AttrType| of 'is_test' in OpDesc can be INT or BOOLEAN

* test=develop

* [LITE][XPU] suppress linkage error
test=develop

* [LITE][XPU] 1. Reorder |identity_dropout_eliminate_pass| before |__xpu__multi_encoder_fuse_pass|; 2. Enhance |__xpu__multi_encoder_fuse_pass|, it works well in more scenarios;
test=develop

* [LITE][XPU] Remove XPUConfig
test=develop