[Onnx] Support Bidirectional RNNs (apache#8337)

* modify lstm to be easily bidirectional

* make it obvious some matriciies are packed via prime notation

* fix var name

* more var names

* add op split

* keyword arg names

* missing implicit cls arg

* deal with extra dimensions

* last of the fixes

* refactor rnn tests to support directions

* bidirectional tests

* test forward results

* go backwards

* more fixes

* reverse tokens on reverse pass

* parameterized directions

* double up activations in bidirect

* slow attribute forgetting

* lstm interface is v. confus

* test forward complete

* add GRU outline

* revisiion2

* why was tehre a not

* gru tests

* missing bounds, copy pasta!

* add comment

* ensure all args fp