Do not require UniqueTogether validation for updates (PATCH) since the update may be partial.

This was a quick workaround when troubleshooting an issue when passing partial data via PATCH to update a resource where `unique_together` constraints were imposed on the model and the serializer validation was trying to enforce the uniqueness even though it only had partial data.

```python
class Extension(models.Model):                                                                                                         
    ext = models.CharField(max_length=255)                                                                               
    domain = models.ForeignKey(Domain)                                          
                                                                                
    class Meta:                                                                 
        db_table = 'extension'                                                  
        unique_together = (("ext", "domain"),)         
```

```python
class ExtensionSerializer(HyperlinkedModelSerializer):                          
    domain = HrefRelatedField(view_name='domain-detail', lookup_field='name', queryset=Domain.objects.all())                                                                       
                    
    class Meta(HyperlinkedModelSerializer.Meta):                                
        model = Extension                                                       
        fields = ('url', 'ext', 'domain')     
```

PATCH http://testserver/v1/extensions/1/ --- {'domain': 'http://testserver/v1/domains/2/'}

Results in KeyError 'ext' because ModelSerializer.get_validators() looks for all fields specific in `unique_together` in the PATCH (partial) update.