Merge pull request #1421 from loadingArbert/wayneliu/switchSwiftPB

resolve PMOpDesc&ProgramOptimize warning

metal/paddle-mobile/paddle-mobile/Src/Program/PMOpDesc.swift

@@ -27,7 +27,7 @@ class PMOpDesc {
             var map: [String : [String]] = [:]
             for opDescVar  in vars {
                 if (canAdd(opDescVar.parameter)) {
-                    map[opDescVar.parameter] = opDescVar.argumentsArray as! [String]
+                    map[opDescVar.parameter] = opDescVar.argumentsArray as? [String]
                 }
             }
             return map

metal/paddle-mobile/paddle-mobile/Src/Program/ProgramOptimize.swift

@@ -292,7 +292,7 @@ class ProgramOptimize<P: PrecisionType> {
       ops.append(node.opDesc!)
     }
     
-    var newProgramDesc = PMProgramDesc.init()
+    let newProgramDesc = PMProgramDesc.init()
     let newBlock = PMBlockDesc.init(inVars: block.vars, inOps: ops)
     newProgramDesc.blocks.append(newBlock)
     return newProgramDesc