function flatMapDeep(collection, iteratee) {
      return baseFlatten(map(collection, iteratee), INFINITY);
    }
This method is like `_.flatMap` except that it recursively flattens the
mapped results.

@static
@memberOf _
@since 4.7.0
@category Collection
@param {Array|Object} collection The collection to iterate over.
@param {Function} [iteratee=_.identity] The function invoked per iteration.
@returns {Array} Returns the new flattened array.
@example

function duplicate(n) {
  return [[[n, n]]];
}

_.flatMapDeep([1, 2], duplicate);
// => [1, 1, 2, 2]