fix(batch): make instanceType optional on multinode jobs

packages/@aws-cdk-testing/framework-integ/test/aws-batch/test/integ.multinode-job-definition.js.snapshot/stack.template.json

@@ -76,7 +76,6 @@
          ]
         },
         "Image": "amazon/amazon-ecs-sample",
-        "InstanceType": "r4.large",
         "ReadonlyRootFilesystem": false,
         "ResourceRequirements": [
          {

packages/@aws-cdk-testing/framework-integ/test/aws-batch/test/integ.multinode-job-definition.ts

@@ -8,7 +8,6 @@ const app = new App();
 const stack = new Stack(app, 'stack');
 
 new batch.MultiNodeJobDefinition(stack, 'SingleContainerMultiNodeJob', {
-  instanceType: InstanceType.of(InstanceClass.R4, InstanceSize.LARGE),
   containers: [{
     startNode: 0,
     endNode: 10,

packages/aws-cdk-lib/aws-batch/README.md

@@ -556,7 +556,7 @@ In particular, the environment variable that tells the containers which one is t
 
 ```ts
 const multiNodeJob = new batch.MultiNodeJobDefinition(this, 'JobDefinition', {
-  instanceType: ec2.InstanceType.of(ec2.InstanceClass.R4, ec2.InstanceSize.LARGE),
+  instanceType: ec2.InstanceType.of(ec2.InstanceClass.R4, ec2.InstanceSize.LARGE), // optional, omit to let Batch choose the type for you
   containers: [{
     container: new batch.EcsEc2ContainerDefinition(this, 'mainMPIContainer', {
       image: ecs.ContainerImage.fromRegistry('yourregsitry.com/yourMPIImage:latest'),