Fix previous force downcasting codes in several places.

Suggestion from: https://goo.gl/RyyFHo

Author: pcpLiu

Source/Serrano/core/symbol.swift

@@ -229,7 +229,7 @@ extension ScalarSymbol {
 		if bindedData == nil {
 			return nil
 		} else {
-			return [self.bindedData as! DataSymbolSupportedDataType]
+			return [self.bindedData!]
 		}
 	}
 }

Source/Serrano/graph/symbols/scalar_symbol.swift

@@ -101,8 +101,12 @@ public class SerranoScalarSymbol: SerranoGraphSymbol, ScalarSymbol {
 			return false
 		}
 
-		self._bindedData = data as! SupportedScalarDataType
-		
+		guard let bindedData = data as? SupportedScalarDataType else {
+			SerranoLogging.errorLogging(message: "Scalar symbol \(self.symbolLabel) was trying to bind to data \(data). But seems this data is not a scalar.",
+				file: "\(#file)", function: "\(#function)", line: "\(#line)")
+			fatalError("Faltal error raised by Serrano. Check log for details.")
+		}
+		self._bindedData = bindedData
 		return true
 	}
 }

Source/Serrano/operators/nn/common/fullyconnected_op.swift

@@ -395,9 +395,19 @@ public class FullyconnectedOperator: ComputableOperator {
 				fatalError("Faltal error raised by Serrano. Check log for details.")
 			}
 			if label == "weight" {
-				self.weight = dataSymbol.bindedData! as! Tensor
+				guard let weightTensor = dataSymbol.bindedData! as? Tensor else {
+					SerranoLogging.errorLogging(message: "Fully connected operator \(self.operatorLabel) is trying to bind to data symbol \(dataSymbol) for weight. But seems this symbol is not a tensor symbol as expected.",
+						file: "\(#file)", function: "\(#function)", line: "\(#line)")
+					fatalError("Faltal error raised by Serrano. Check log for details.")
+				}
+				self.weight = weightTensor
 			} else {
-				self.bias = dataSymbol.bindedData! as! Tensor
+				guard let biasTensor = dataSymbol.bindedData! as? Tensor else {
+					SerranoLogging.errorLogging(message: "Fully connected operator \(self.operatorLabel) is trying to bind to data symbol \(dataSymbol) for bias. But seems this symbol is not a tensor symbol as expected.",
+						file: "\(#file)", function: "\(#function)", line: "\(#line)")
+					fatalError("Faltal error raised by Serrano. Check log for details.")
+				}
+				self.bias = biasTensor
 			}
 		}
 	}

Source/Serrano/operators/nn/convolution/conv_op.swift

@@ -358,8 +358,13 @@ public class ConvOperator2D: ComputableOperator {
 				fatalError("Faltal error raised by Serrano. Check log for details.")
 			}
 
-			self.weight = dataSymbol.bindedData! as! Tensor
-			
+//			self.weight = dataSymbol.bindedData! as! Tensor
+			guard let weightTensor = dataSymbol.bindedData! as? Tensor else {
+				SerranoLogging.errorLogging(message: "Cov2D operator \(self.operatorLabel) was trying to bind to data symbol \(dataSymbol). But seems this symbol is not a tensor symbol as expected.",
+					file: "\(#file)", function: "\(#function)", line: "\(#line)")
+				fatalError("Faltal error raised by Serrano. Check log for details.")
+			}
+			self.weight = weightTensor
 		}
 	}
 

docs/Contribution/Contribution.md

@@ -0,0 +1,6 @@
+# Contribution Guide 
+
+Please contribute to improve Serrano :hot_pepper: helping make it better.
+Before opening an issue or submitting a pull request, please read through this guide.
+
+## Issue submitting