Reset backend

[alecandido]

We may want to define a reset() function (or something similar) to achieve this purpose.

Or, even better, just use set_backend() and set_transpiler() without arguments (that could default to None), in order to reset (i.e. to set the attributes to None, as you're doing here, and do nothing else).

Originally posted by @alecandido in #1440 (comment)