Add optional warning for backwards incompatible broadcast.

Setting torch.utils.backcompat.broadcast.warning.enabled=True
will cause Python warnings in the case where broadcast occurs
but previously 1-d view style pointwise ops occured.

torch/csrc/utils.cpp

@@ -551,3 +551,13 @@ void THPPointer<THPGenerator>::free() {
 }
 
 template class THPPointer<THPGenerator>;
+
+static bool backCompatBroadcastWarn = false;
+
+void setBackCompatBroadcastWarn(bool warn) {
+  backCompatBroadcastWarn = true;
+}
+
+bool getBackCompatBroadcastWarn() {
+  return backCompatBroadcastWarn;
+}