Avoid producing presentKey/presentValue outputs if pastKey/pastValue …

…don't exists.

js/web/lib/wasm/jsep/webgpu/ops/attention.ts

@@ -371,7 +371,7 @@ const createAttentionProbsProgramInfo = (
 ) => {
   const totalSequenceLength = pastSequenceLength + parameters.kvSequenceLength;
   const probsShape = [parameters.batchSize, parameters.numHeads, parameters.sequenceLength, totalSequenceLength];
-  const presentKey = parameters.kvNumHeads === undefined && context.outputCount > 1;
+  const presentKey = parameters.kvNumHeads === undefined && context.outputCount > 1 && pastKey;
   const presentKeyShape = presentKey
     ? [parameters.batchSize, parameters.numHeads, totalSequenceLength, parameters.headSize]
     : undefined;
@@ -534,7 +534,7 @@ const createVxAttentionScoreProgramInfo = (
   const totalSequenceLength = pastSequenceLength + params.kvSequenceLength;
   const nReps = params.nReps ? params.nReps : 1;
   const repeatedVHiddenSize = params.vHiddenSize * nReps;
-  const presentValue = params.kvNumHeads == null && context.outputCount > 1;
+  const presentValue = params.kvNumHeads == null && context.outputCount > 1 && pastValue;
   const presentValueShape = presentValue
     ? [params.batchSize, params.numHeads, totalSequenceLength, params.headSize]
     : undefined;
@@ -668,7 +668,8 @@ export const applyAttention = (
   parameters: AttentionParameters,
   attributes: AttentionAttrs,
 ) => {
-  const outputCount = context.outputCount;
+  // Assumption  is that presentKey/presentValue exists only if pastKey/pastValue exists
+  const outputCount = Math.min(context.outputCount, 1 + (pastKey ? 1 : 0) + (pastValue ? 1 : 0));
   const pastSequenceLength = parameters.kvNumHeads !== undefined || outputCount > 1 ? parameters.pastSequenceLength : 0;
   const totalSequenceLength = pastSequenceLength + parameters.kvSequenceLength;