title: Django admin TabularInline with custom form
body: <p>I want to use TabularInline for my model-admin with a custom form and <code>extra=3</code>. But the problem is that when I hit <strong>save</strong> in my admin page, the form gives validation error on empty extras that I didn't use and can't save the models I wanted to create. I read the docs in django and can't quite understand the solution to my problem.</p>
  
  <p>The reason I want to use a custom form is that I'm using custom widgets for some of the fields. So there may be a better solution for my needs than using a custom form which I'm not aware of.
  I'm using Django v2.0 and here's the code for my admin:</p>
  
  <pre><code>class BingilForm(forms.ModelForm):
      class Meta:
          model = Bingil
          fields = '__all__'
  
          widgets = {
              'position_top': RangeInput(),
              'position_left': RangeInput(),
              'background_color': ColorInput(),
              'foreground_color': ColorInput(),
          }
  
  class InlineVideoBingil(admin.TabularInline):
      model = Bingil
      fk_name = 'video'
      form = BingilForm
      extra = 3
      show_change_link = True
  </code></pre>