[hotfix] Discard watermarks when feed a datastream into iteration body

This closes #223.
diff --git a/flink-ml-iteration/src/main/java/org/apache/flink/iteration/Iterations.java b/flink-ml-iteration/src/main/java/org/apache/flink/iteration/Iterations.java
index 6e6ff9d..8f71677 100644
--- a/flink-ml-iteration/src/main/java/org/apache/flink/iteration/Iterations.java
+++ b/flink-ml-iteration/src/main/java/org/apache/flink/iteration/Iterations.java
@@ -85,6 +85,9 @@
  * <p>The limitation of constructing the subgraph inside the iteration body could be refer in {@link
  * IterationBody}.
  *
+ * <p>Note that the iteration framework cannot deal with watermarks correctly for now. It should be
+ * resolved by FLINK-31373.
+ *
  * <p>An example of the iteration is like:
  *
  * <pre>{@code
diff --git a/flink-ml-iteration/src/main/java/org/apache/flink/iteration/operator/InputOperator.java b/flink-ml-iteration/src/main/java/org/apache/flink/iteration/operator/InputOperator.java
index b6908ff..604715f 100644
--- a/flink-ml-iteration/src/main/java/org/apache/flink/iteration/operator/InputOperator.java
+++ b/flink-ml-iteration/src/main/java/org/apache/flink/iteration/operator/InputOperator.java
@@ -22,6 +22,7 @@
 import org.apache.flink.streaming.api.operators.AbstractStreamOperator;
 import org.apache.flink.streaming.api.operators.ChainingStrategy;
 import org.apache.flink.streaming.api.operators.OneInputStreamOperator;
+import org.apache.flink.streaming.api.watermark.Watermark;
 import org.apache.flink.streaming.runtime.streamrecord.StreamRecord;
 
 /** Input operator that wraps the user record into {@link IterationRecord}. */
@@ -46,4 +47,9 @@
         reusable.getValue().setValue(streamRecord.getValue());
         output.collect(reusable);
     }
+
+    @Override
+    public void processWatermark(Watermark mark) {
+        // TODO: FLINK-31373 Support processing watermarks in iterations.
+    }
 }