Support multiple inheritance in torch.distributions (#16772)

Summary:
This adds calls to `super().__init__()` in three classes in torch.distributions.

This is needed when `Distribution` and `Transform` objects are used with multiple inheritance, as e.g. combined with `torch.nn.Module`s. For example
```py
class MyModule(torch.distributions.Transform, torch.nn.Module):
    ...
```
cc  martinjankowiak esling who have wanted to use this pattern, e.g. in #16756
Pull Request resolved: #16772

Differential Revision: D13978633

Pulled By: soumith

fbshipit-source-id: 8bc6cca1747cd74d32135ee2fe588bba2ea796f1

torch/distributions/constraint_registry.py

@@ -82,6 +82,7 @@ class ConstraintRegistry(object):
     """
     def __init__(self):
         self._registry = {}
+        super(ConstraintRegistry, self).__init__()
 
     def register(self, constraint, factory=None):
         """

torch/distributions/distribution.py

@@ -34,6 +34,7 @@ def __init__(self, batch_shape=torch.Size(), event_shape=torch.Size(), validate_
                     continue  # skip checking lazily-constructed args
                 if not constraint.check(getattr(self, param)).all():
                     raise ValueError("The parameter {} has invalid values".format(param))
+        super(Distribution, self).__init__()
 
     def expand(self, batch_shape, _instance=None):
         """

torch/distributions/transforms.py

@@ -83,6 +83,7 @@ def __init__(self, cache_size=0):
             self._cached_x_y = None, None
         else:
             raise ValueError('cache_size must be 0 or 1')
+        super(Transform, self).__init__()
 
     @property
     def inv(self):