Independent constraint (#50547)

Summary:
Addresses https://github.com/pytorch/pytorch/issues/50496

This fixes a number of inconsistencies in torch.distributions.constraints as used for parameters and supports of probability distributions.
- Adds a `constraints.independent` and replaces `real_vector` with `independent(real, 1)`. (this pattern has long been used in Pyro)
- Adds an `.event_dim` attribute to all constraints.
- Tests that `constraint.check(data)` has the correct shape. (Previously the shapes were incorrect).
- Adds machinery to set static `.is_discrete` and `.event_dim` for `constraints.dependent`.
- Fixes constraints for a number of distributions.

## Tested
- added a new check to the constraints tests
- added a new check for `.event_dim`

cc fehiepsi feynmanliang stefanwebb

Pull Request resolved: https://github.com/pytorch/pytorch/pull/50547

Reviewed By: VitalyFedyunin

Differential Revision: D25918330

Pulled By: neerajprad

fbshipit-source-id: a648c3de3e8704f70f445c0f1c39f2593c8c74db

torch/distributions/binomial.py

@@ -67,7 +67,7 @@ class Binomial(Distribution):
     def _new(self, *args, **kwargs):
         return self._param.new(*args, **kwargs)
 
-    @constraints.dependent_property
+    @constraints.dependent_property(is_discrete=True)
     def support(self):
         return constraints.integer_interval(0, self.total_count)