models.py

class Registration(TimeStampedModel):
  ....
  tutor_m = models.ForeignKey(
        "universities.Tutor",
        on_delete=models.CASCADE,
    )
  ....

signals.py

from django.db.models.signals import post_delete
from django.dispatch import receiver

from .models import Registration


# TODO when deleting multiple registration with same tutor, an error occurs
@receiver(post_delete, sender=Registration)
def registration_delete(sender, instance, *args, **kwargs):
    if not (Registration.objects.filter(tutor_m=instance.tutor_m).exists():
       try:
          instance.tutor_m.delete()
       except Tutor.DoesNotExist:
          pass
    # if len(Registration.objects.filter(tutor_m=instance.tutor_m)) == 1:
    #     instance.tutor_m.delete()