Revert "Explicit broadcast"

This reverts commit b7d1da2.

Author: YingboMa

src/rule_definition_tools.jl

@@ -211,12 +211,12 @@ function propagation_expr(Δs, ∂s)
     ∂s = map(esc, ∂s)
     n∂s = length(∂s)
 
-    ∂_mul_Δs = ntuple(i->:($(∂s[i]) .* $(Δs[i])), n∂s)
+    ∂_mul_Δs = ntuple(i->:($(∂s[i]) * $(Δs[i])), n∂s)
 
-    # avoiding the extra `.+` operation, it is potentially
+    # avoiding the extra `+` operation, it is potentially
     # expensive for vector mode AD
     sumed_∂_mul_Δs = if n∂s > 1
-        :(.+($(∂_mul_Δs...)))
+        :(@. +($(∂_mul_Δs...)))
     else
         ∂_mul_Δs[1]
     end