Identity tests fail randomly: need to set RNG state in tests

[alvorithm]

Some of the tests check that the forward and backward passes of a transform compose to give the identity function. there are numerical checks on this with specified tolerances, often 1e-4. This can succeed or fail depending on RNG state, which is not currently set by any tests.

@dgreenberg this is your issue I transferred from pyknós (nflows is the right home); I had to create it anew because transferring of issues from private to public repo is not supported.