Backend-agnostic ABC

[willGraham01]

Concerns #8 |

Adds a base class for backend-agnostic objects that will be needed in the package.

A BackendAgnostic class is setup such that it must have certain attributes, and a reference to some _backend_obj that will actually be doing the heavy-lifting for accessing / computing these attributes. This allows us to define methods on the BackendAgnostic class that conform to our expected frontend behaviours / syntax, and have these methods translate the arguments provided by the frontend into the arguments that need to be passed to the _backend_obj.

The most obvious example of these objects will be Distributions - these are required to have a sample method, but different backends (jax.random, distrax, numpyro, etc) have different syntaxes for sampling from their distributions. A BackendAgnostic distribution necessitates having a sample method, but for the different backends it would be implemented differently;

class Distribution(BackendAgnostic):

    @property
    def _frontend_provides(self):
        return ("sample",)

class JaxDistribution(Distribution):

    def sample(self, rng_key, sample_shape):
        return self._backend_obj.sample(rng_key, sample_shape)

class NumPyroDistribution(Distribution):

    def sample(self, rng_key, sample_shape):
        return self._backend_obj.sample(key=rng_key, sample_shape=sample_shape)

NOTE: This will be one of several "breakdown" PRs that all-together form #23.

NOTE 2: In light if #24, the jax-specific rng_keys should eventually be replaced with some backend-agnostic class that accepts a .split method.