[{"nestedMap":{"1001":{"0":"A","1":"B","2":"C","3":"D"},"1002":{"0":"E","1":"F","2":"G","3":"H"}}}]

db.collection.aggregate([{$set:{nestedMap:{$objectToArray:"$nestedMap"}}},{$project:{nestedMap:{$map:{input:"$nestedMap",in:{k:"$$this.k",v:{$objectToArray:"$$this.v"}}}},_id:0}}])