Implements stateless entropy models.

- Renames `no_variables` argument to `stateless`. This prevents creation of
  `tf.Variable` objects for the range coding tables and other data necessary
  to compress/decompress. (Stateless in the sense that `Variable` objects are
  used to represent TensorFlow state.)
- Removes the alternative initializer in `from_config`. The default
  implementation of this class method calls `__init__` with the keyword
  arguments stored in the config, so the necessary logic is now in `__init__`.
- Harmonizes some of the shared keyword arguments and their docstrings across
  the class hierarchy.

PiperOrigin-RevId: 368293268
Change-Id: Id8d9bb509e08d8977b44829d30bb76ac93fb10d5

Author: Johannes Ballé