Gamma distribution and the derived distributions (Beta, Dirichlet, Student's...

Gamma distribution and the derived distributions (Beta, Dirichlet, Student's t, inverse Gamma) are fully reparameterized.

For every distribution, the changes are:
* Set reparameterization_type to FULLY_REPARAMETERIZED.
* Add a note about reparameterization and an example to the docstring.
* Add a test that ensures that the gradients exist.

Additional changes:
* Fix docstring and test in TFP that assume that Gamma is not reparameterized. We simply replace Gamma with Bernoulli :)
* Fix paths to modules in docstrings.

PiperOrigin-RevId: 201691205