As Adrian suggested
if you choose templateBuilder.fastest() using the aws-ec2 provider, a cluster compute instance will be used and setup placement group automatically.
if you want to specify placementgroup manually use AWSEC2TemplateOptions.placementGroup
'fastest' function could be a nice thing to expose, but it is not the solution to this problem. I think we have to go with calling placementGroup explicitly, but placementGroup is not exposed in the TemplateOptions interface. Any suggestions?