title: __init__() missing 1 required positional argument: 'model_field'
body: <p>I am trying to use Nested Serializer, if I remove the "product=ProductSerializer()" from serializer A, the code runs sucessfully, else it gives the below error.</p>
  
  <p>How come the SizeSerializer is working but Productserializer don't.</p>
  
  <p><strong>model A</strong></p>
  
  <pre><code>class SKU(models.Model):
      size = models.ForeignKey(Size,on_delete=models.CASCADE)
      product = models.ForeignKey(Product,on_delete=models.CASCADE)
  
      number = models.CharField(max_length=100,null=True,blank=True)
      price = models.IntegerField(null=True,blank=True)
      parent_sku = models.CharField(max_length=100,null=True,blank=True)
      active = models.BooleanField(default=True)
  </code></pre>
  
  <p><strong>model B</strong></p>
  
  <pre><code>class Product(models.Model):      
      name = models.CharField(max_length=200,null=True,blank=True)
      active = models.BooleanField(default=True)
  </code></pre>
  
  <p><strong>serializer A</strong></p>
  
  <pre><code>class SKU_Serializer(serializers.ModelSerializer):
      size    = SizeSerializer()
      product = ProductSerializer()
  
      class Meta:
          model = SKU
          exclude = ('created_at','updated_at')
  </code></pre>
  
  <p><strong>serializer B</strong></p>
  
  <pre><code>class ProductSerializer(serializers.ModelField):
  
      class Meta:
          model = Product
          exclude = ('created_at','updated_at')
  </code></pre>
  
  <p><strong>json</strong></p>
  
  <pre><code>{
          "id": 1,
          "size": {
              "id": 1,
              "name": "XL",
              "active": true
          },
          "number": "Muriel Treppas",
          "price": 899,
          "parent_sku": "Muriel",
          "active": true,
          "product": 1
      },
  </code></pre>
  
  <p><strong>output(error)</strong></p>
  
  <pre><code>  File "D:\1___DEVELOPMENT\Clothing\hoplet\sku\views.py", line 6, in &lt;module&gt;
      from sku.serializers import SKU_Serializer
    File "D:\1___DEVELOPMENT\Clothing\hoplet\sku\serializers.py", line 8, in &lt;module&gt;
      class SKU_Serializer(serializers.ModelSerializer):
    File "D:\1___DEVELOPMENT\Clothing\hoplet\sku\serializers.py", line 10, in SKU_Serializer
      product = ProductSerializer()
  TypeError: __init__() missing 1 required positional argument: 'model_field'
  </code></pre>