Unroll the reduction loop once to get the container, and remove the Compiler call

Author: YingboMa

src/ensemble/basic_ensemble_solve.jl

@@ -98,24 +98,20 @@ function __solve(prob::AbstractEnsembleProblem,
     return EnsembleSolution(_u,elapsed_time,true)
   end
 
-  if prob.u_init === nothing && prob.reduction === DEFAULT_REDUCTION
-    batchrt = Core.Compiler.return_type(batch_function,Tuple{UnitRange{Int64}})
-  else
-    batchrt = Any
-  end
-  u = batchrt[]
-
-  converged = false
+  converged::Bool = false
 
-  elapsed_time = @elapsed for i in 1:num_batches
+  batch_data = batch_function(I)
+  u = similar(batch_data, 0)
+  u,converged = prob.reduction(u,batch_data,I)
+  elapsed_time = @elapsed for i in 2:num_batches
+    converged && break
     if i == num_batches
       I = (batch_size*(i-1)+1):trajectories
     else
       I = (batch_size*(i-1)+1):batch_size*i
     end
     batch_data = batch_function(I)
     u,converged = prob.reduction(u,batch_data,I)
-    converged && break
   end
 
   u = reduce(vcat, u)