title: how to use custom user model for django-graphql-auth
body: <p>I am using graphql in my project and want to generate token while registration. Even though django-graphql-auth has all the written mutation, it uses a different user model. But I want to use my custom user model. What should I do?
  This is my user model</p>
  
  <pre><code>class CustomUser(AbstractUser):
  """
  Customized User Model
  """
  email = models.EmailField(blank=True, null=True,
                            max_length=256, unique=True)
  mobile_no = models.CharField(
      max_length=20, blank=True, null=True, unique=True)
  verification_code = models.CharField(max_length=6)
  code_valid_till = models.DateTimeField(blank=True, null=True)
  
  timezone = models.CharField(max_length=40, blank=True, null=True)
  country = models.CharField(max_length=20, default='BD', choices=COUNTRIES)
  
  pin_verified = models.BooleanField(default=False)
  email_verified = models.BooleanField(default=False)
  
  modified_on = models.DateTimeField('date modified', auto_now=True)
  created_on = models.DateTimeField(auto_now_add=True)
  
  # For password reset
  reset_verification_code = models.CharField(
      max_length=6, blank=True, null=True)
  reset_code_valid_till = models.DateTimeField(blank=True, null=True)
  reset_request = models.BooleanField(default=False)
  reset_valid_till = models.DateTimeField(blank=True, null=True)
  
  class Meta:
      verbose_name_plural = "User"
  
  def __str__(self):
      if not self.is_anonymous:
          return "{} - {}".format(self.mobile_no, self.email)
      else:
          return "Anon"
  
  @property
  def get_full_name(self):
      return super().get_full_name()
  
  def get_all_permissions(self, obj=None):
      return super().get_all_permissions(obj=obj)
  
  def send_email(self, subject, message, to_email: list, from_email=None, **kwargs):
      send_mail_to_user(subject, message, from_email, to_email, **kwargs)
  </code></pre>