[OPS] Added a check for 'stack' that avoids an ugly segfault.

Author: eaplatanios

RELEASE.md

@@ -1,3 +1,8 @@
+# 0.6.2
+
+- Added a check for the `stack` operation that avoids an 
+  ugly segfault.
+
 # 0.6.1
 
 - `Session.run` no longer locks.

modules/api/src/main/scala/org/platanios/tensorflow/api/learn/SessionCreator.scala

@@ -70,7 +70,7 @@ case class ChiefSessionCreator(
     if (extraInitOps.isEmpty)
       builtSessionScaffold.initOp
     else
-      ControlFlow.group(extraInitOps.toSet + builtSessionScaffold.localInitOp, name = "Init").asUntyped
+      ControlFlow.group(extraInitOps.toSet + builtSessionScaffold.initOp, name = "Init").asUntyped
   }
 
   override protected lazy val localInitOp: UntypedOp = {

modules/api/src/main/scala/org/platanios/tensorflow/api/ops/basic/Manipulation.scala

@@ -300,6 +300,7 @@ trait Manipulation {
       axis: Int = 0,
       name: String = "Stack"
   ): Output[T] = {
+    require(inputs.nonEmpty, "Cannot stack an empty Seq[Output].")
     Op.Builder[Seq[Output[T]], Output[T]](
       opType = "Pack",
       name = name,

modules/api/src/main/scala/org/platanios/tensorflow/api/tensors/ops/Basic.scala

@@ -137,6 +137,7 @@ trait Basic {
       inputs: Seq[Tensor[T]],
       axis: Int = 0
   ): Tensor[T] = {
+    require(inputs.nonEmpty, "Cannot stack an empty Seq[Tensor].")
     Tensor.fromNativeHandle[T](NativeTensorOpsBasic.pack(
       executionContext.value.nativeHandle, inputs.map(_.nativeHandle).toArray, axis))
   }