Here is the implementation. You have to implement these three classes to achieve the desire result. I created a table to store the Opportunity Sum.

global class AggregateResultIterator implements Iterator<AggregateResult> {
   AggregateResult [] results {get;set;}
   // tracks which result item is returned
   Integer index {get; set;} 
         
   global AggregateResultIterator() {
      index = 0;
	  // Fire query here to load the results
      String query = 'SELECT SUM(Amount)OpptySum FROM Opportunity';
      results = Database.query(query);            
   } 
   
   global boolean hasNext(){ 
      return results != null && !results.isEmpty() && index < results.size(); 
   }    
   
   global AggregateResult next(){        
      return results[index++];            
   }       
}

 

global class AggregateResultIterable implements Iterable<AggregateResult> {
   global Iterator<AggregateResult> Iterator(){
      return new AggregateResultIterator();
   }
}

global class IterableAggregator implements Database.Batchable<AggregateResult> {
    integer counter = 0;
    integer opptsumx = 0;
    list<testopptysum__c> oplst = new list<testopptysum__c>();
    testopptysum__c op; 
    global Iterable<AggregateResult> start(Database.batchableContext info){
        // just instantiate the new iterable here and return
        return new AggregateResultIterable();
    }

    global void execute(Database.BatchableContext BC, List<AggregateResult> scope){
        for (AggregateResult so : scope)  {
          AggregateResult ar = (AggregateResult) so;
          
            
          counter = Integer.valueOf(ar.get('OpptySum'));
            opptsumx = opptsumx + counter;
            
          // process the results
        }
		op = new testopptysum__c();
        op.opptsum__c = opptsumx;
        insert op;        
    }

    global void finish(Database.BatchableContext BC){
        
    }   
}

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