謎だったのですが、ECS_AL2 か ECS_AL2_NVIDIA のようでした。
（Web Consoleから作成した後にJSONを確認したところ）

resource "aws_batch_compute_environment" "sample" {
  compute_environment_name = "sample"

  compute_resources {
    instance_role = aws_iam_instance_profile.ecs_instance_role.arn

    instance_type = [
      "c4.large",
    ]

    max_vcpus = 16
    min_vcpus = 0

    security_group_ids = [
      aws_security_group.sample.id,
    ]

    subnets = [
      aws_subnet.sample.id,
    ]

    type = "EC2"

    ec2_configuration {
      image_type = "ECS_AL2 or ECS_AL2_NVIDIA"
    }
  }

  service_role = aws_iam_role.aws_batch_service_role.arn
  type         = "MANAGED"
  depends_on   = [aws_iam_role_policy_attachment.aws_batch_service_role]
}