It would be great to add some optional attribute to form element or div with ng-form attribute which will say that validation errors of current form shouldn't affect validation of parent form.
Here is what I mean:

<div ng-form="parentForm">
  <input...>some valid input</input>
  <div ng-form="childForm" omit-parent-validation="true">
    <input...>some INVALID input</input>
  </div>
</div>

Result:
parentForm.$valid -> true, childForm.$valid -> false
Same logic may be applied for
parentForm.$pristine -> true, childForm.$pristine -> false