Introduce `BeliefPropagationProblem` wrapper to hold the cache and the
error `diff` field.

Also simplifies some kwargs wrangling.

…be set from another cache

Also includes some fixes to the way `TensorNetwork` types are
constructed based on index structure.

for more information, see https://pre-commit.ci

…instead of trying to operate on existing graphs

The reason for this is:
- One only cares about the edges of the input graph
- A simple graph cannot be used as it "forgets" its edge names resulting
in recursion
- As shown with `TensorNetwork`, removing edges may not always be
defined.

…s from an array.

…a order-0 array

This was caused by the change to the `cache` being backed by a directed
graph.

Defaults to one constructed via `randname`.

for more information, see https://pre-commit.ci

This local refers to a index name not an index itself.