Transparent mode now working.

ScriptableRenderPipeline/LightweightPipeline/Shaders/LightweightPipelineCore.cginc

 
 half4 OutputColor(half3 color, half alpha)
 {
-#ifdef _ALPHABLEND_ON
+#if defined(_ALPHABLEND_ON) || defined(_ALPHAPREMULTIPLY_ON)
     return LIGHTWEIGHT_LINEAR_TO_GAMMA(half4(color, alpha));
 #else
     return half4(LIGHTWEIGHT_LINEAR_TO_GAMMA(color), 1);

ScriptableRenderPipeline/LightweightPipeline/Shaders/LightweightStandardShader.shader

 #else
                 half3 diffColor = SpecularSetup(uv, albedo, alpha, specColor, smoothness, oneMinusReflectivity);
 #endif
+
+                diffColor = PreMultiplyAlpha(diffColor, alpha, oneMinusReflectivity, /*out*/ alpha);
+
                 // Roughness is (1.0 - smoothness)²
                 half perceptualRoughness = 1.0h - smoothness;