fix: support tensorflow 2.2.0 rc (explosion#331)

* fix: support tensorflow 2.2.0 rc

 - update tensorflow version pin to include versions < 2.3.0
 - check for existence of private `_make_train_function` before invoking

* fix: don't add cupyx as class member

 - there's a test that asserts that all the Ops classes have to have the same attributes. It started failing by adding self.xp2 to cupyops and not the base class.
  - another way to fix this would be to add it to the base class

* chore: better tensorflow 2.2.x fix

 - make_train_function was made public, not removed

* chore: make _xp2 private on CupyOps

set nI/nO on chained models (explosion#324)

* set nI/nO on chained models

* check None to prevent mypy error

* fix things in chain instead

* fix formatting

Set version to v7.4.0

Set version to v8.0.0a1

Merge branch 'develop'

Update setup.cfg and dev version

Set version to v8.0.0.dev3

Set v to 8.0.0v2

Set version to v8.0.0.dev1

Add with_flatten (explosion#244)

Add with_flatten