Make tf.Module extend ABCMeta to make abstract subclasses trivial.

```
import abc

class AbstractModule(tf.Module):
  @abc.abstractmethod
  def __call__(self, inputs, is_training=False):
    pass
```

You can do this more verbosely if you prefer:

```
import abc
import six

@six.add_metaclass(abc.ABCMeta)
class AbstractModule(tf.Module):
  @abc.abstractmethod
  def __call__(self, inputs, is_training=False):
    pass
```

PiperOrigin-RevId: 234186396