[Paddle-TRT] allow plugin fall back to fp16 when int8 (#50554)

* allow fall back to fp16 when int8

* refine code

* refine code

* refine code

paddle/fluid/inference/tensorrt/engine.h

@@ -358,7 +358,9 @@ class TensorRTEngine {
   bool WithFp16() {
     bool enable_fp16 = (precision_ == AnalysisConfig::Precision::kHalf);
     bool support_fp16 = infer_builder_->platformHasFastFp16();
-    return enable_fp16 && support_fp16;
+    // below is consistent with setFlag in engine.cc
+    bool fall_back_fp16 = WithInt8() && !use_dla_;
+    return (enable_fp16 || fall_back_fp16) && support_fp16;
   }
 
   bool WithInt8() {