Make WhereNode work in the light of gaps.

Source/ComputationNetworkLib/ReshapingNodes.cpp

@@ -79,19 +79,24 @@ template <class ElemType>
     outMBLayout->InitAsPackedSequences(SequenceLengthVector(sequences, indexSequences), /*temp*/m_placementBuffer, /*temp*/m_rowAllocationsBuffer);
     // copy to output
     vector<ElemType> buf(outMBLayout->GetNumCols(), numeric_limits<ElemType>::quiet_NaN()); // STL cannot easily avoid initializing, so we might as well init with NaN for gaps
-    for (size_t i = 0; i < sequences.size(); i++)
+    for (size_t i = 0, j = 0; i < sequences.size();)
     {
-        let& seq = outMBLayout->GetAllSequences()[i];
-        if (seq.seqId == GAP_SEQUENCE_ID) // gaps will keep the NaN
+        if (sequences[i].seqId == GAP_SEQUENCE_ID) // gaps will keep the NaN
+        {
+            ++i;
+            continue;
+        }
+        let& seq = outMBLayout->GetAllSequences()[j];
+        if (seq.seqId == GAP_SEQUENCE_ID) // When would we see this?
+        {
+            ++j;
             continue;
+        }
         let& indexSequence = indexSequences[i];
         for (size_t t = 0; t < seq.GetNumTimeSteps(); t++)
             buf[outMBLayout->GetColumnIndex(seq, t)] = (ElemType)indexSequence[t];
-        /*
-        BUGBUG: a debug assert when doing this:
-        TakeRight (N, x) = BS.Sequences._Take (FutureValue, N, x)
-        Last(x) = TakeRight(1, x)
-        */
+        ++i;
+        ++j;
     }
     // the result will be kept in CPUDEVICE, since most likely we will access it again in PackedIndexNode
     Value().TransferToDeviceIfNotThere(CPUDEVICE, /*isBeingMoved=*/ true, /*emptyTransfer=*/ true, /*updatePreferredDevice=*/ true);