Add basic heuristics

Author: Avik Pal

src/nnpack/performance.jl

@@ -1,4 +1,16 @@
 function select_threadpool(cdims::DenseConvDims, batch_size::Int)
+    inp_size = input_size(cdims)[1] 
+    if batch_size >= 32
+        return shared_threadpool_dict[4][]
+    elseif batch_size >= 16 && inp_size >= 64
+        return shared_threadpool_dict[4][]
+    elseif inp_size <= 32
+        return C_NULL
+    elseif inp_size >= 128
+        return shared_threadpool_dict[4][]
+    elseif inp_size * batch_size >= 256
+        return shared_threadpool_dict[4][]
+    end    
     return C_NULL
 end